You're asking for which is more powerful - is that really what you care about, or do you just want an interesting character? The number one rule of multiclassing is "never lose spellcasting levels." You've already pretty depowered the character as a Rog3/WW1 even with Magical Knack. I imagine none of the heavy CharOp folks have answered this question yet because your initial build has already provoked them into running about their residences screaming like enraged howler monkeys (rogue, 1 strike; losing spellcasting levels, 2 strikes). If you're looking for superpower, the train has left the station. But if you just want an interesting character to play, read on (though you really should specify what it is you want your character to be able to do/be like...)
In isolation there's a legitimate sorcerer vs witch debate, but if you're a third level rogue who has taken one level in witch, taking anything other than more witch is a severe power compromise - the "third strike." You're getting +2 CL in one class from Magical Knack and then if you were to switch, effectively taking -1 spellcasting level - pretty much losing as much as you're getting. So in this case, "definitely Witch."
As you move on, instead of Sorcerer I'd stick with White-haired Witch (seems like it synergizes well with rogue anyway), or go into Arcane Trickster after a couple levels in Witch - it'll keep full casting progression and is designed to highly synergize with rogue. But never lose a spellcasting level again. Look at it this way, if you were to switch to Sorcerer and be like "woot I want to throw spells", at level 10 you're barely going to be throwing fireballs when normal level 10 spellcasters are really melting faces.
There was a character in my last Pathfinder game who was a Rogue 2/Shadow Oracle 9, that worked out OK (he had a limited times a day super backstab ability and had oracle-boosted stealth stats) so it synergized with rogue well, plus invisibility and major image). So you can multiclass, and even use rogue, but definitely stay away from even more multiclassing. Pathfinder made specific design choices to back away from 3.5's "combination of 6 classes for optimization syndrome" and usually staying single-class is as strong if not stronger than a combo, and the more combo you put in generally the greater a disadvantage you'll have over your comrades.
As written, the witch archetype beast-bonded's supernatural ability twin souls kills a possessed foe
The spell magic jar has a series of clauses making missteps extremely dangerous, both for the possessor and the possessed, but the beast-bonded witch's supernatural ability twin souls changes that effect with three parenthetical words. It says that
...if the witch or her familiar is gravely injured or about to die, the soul of the dying one immediately transfers to the other’s body. The two souls share the surviving body peaceably, can communicate freely, and both retain their ability to think and reason. The host may allow the guest soul to take over the body temporarily or reclaim it as a move action. They can persist in this state indefinitely, or the guest can return to its own body (if available) by touch, transfer into a suitable vessel (such as a clone), or take over another body as if using magic jar (with no receptacle).
Emphasis mine. First, for the ability to activate, the GM must determine that either the witch or the familiar is either gravely injured or about to die (which means, I assume, the creature has the condition dying but ask the GM).1
Assuming the witch activates the special ability twin souls by whittling himself to dying 1 point of damage at a time (the witch doing this to himself rather than to his familiar because the witch needs the familiar to prepare spells), the witch then inhabits the familiar's body as per the description of the ability twin souls and can use an effect like the spell magic jar "with no receptacle."2
What this parenthetical means is deeply unclear. This 2012 thread offers one interpretation. Below I present an alternative that combines the special ability twin souls and the spell magic jar stripped of all references to the receptacle. Such an ability reads...
You can attempt to take control of a nearby body, forcing its soul out of its body, killing it if it has nowhere to go. The spell ends when you send your soul back to your own body.
Attempting to possess a body is a full-round action. It is blocked by protection from evil or a similar ward. You possess the body and force the creature's soul out unless the subject succeeds on a Will save. Failure to take over the host leaves your life force in the current body, and the target automatically succeeds on further saving throws if you attempt to possess its body again.
If you are successful, your life force occupies the host body, and the host's life force is pushed out. You keep your Intelligence, Wisdom, Charisma, level, class, base attack bonus, base save bonuses, alignment, and mental abilities. The body retains its Strength, Dexterity, Constitution, hit points, natural abilities, and automatic abilities. A body with extra limbs does not allow you to make more attacks (or more advantageous two-weapon attacks) than normal. You can't choose to activate the body's extraordinary or supernatural abilities. The creature's spells and spell-like abilities do not stay with the body.
If the host body is slain, you die, and the life force of the host departs (it is dead). Any life force with nowhere to go is treated as slain.
If the spell ends while you are in a host, you return to your body (or die, if it is out of range of your current position).
Alternatively, you can take a move action to touch your body to return your life force to it.
...And while that seems crazy, it's not quite as crazy as it sounds. Yes, this magic jar effect is a save-or-die, but a beast-bound witch must be at least level 10 to use this ability, which means save-or-die effects have been available for at least 3 levels (e.g. phantasmal killer). Further, the witch had to render himself dying first to have access to the ability. Also, once the magic jar effect is used successfully, the only way the witch can use the ability again is by rendering the new body dying again, transferring to the familiar, and finding another body to possess; the ability doesn't appear to work while the witch is possessing a creature, but ask the DM. Finally, the duration of the magic jar effect remains: it's 1 hour/level, and the only longer-lasting switch the witch can make is to his own body or a "suitable vessel," a term the GM must also clearly define.
(Remember, too, that once the witch fails to possess an individual, the witch can never possess that individual. That might be a hassle.)
Officially, however, what the ability should do is mysterious and in the GM's hands. Talk with the GM first before attempting to exploit it. Also, I urge you to press the FAQ button on this post—as I did—to encourage Paizo's development team to address the beast-bound witch's twin souls ability in a future FAQ.
1 This GM would rule that an effect that straight-up, flat-out kills the witch or familiar still leaves the witch ding-dong dead, and the twin souls ability goes unused. Being gravely injured or about to die is a thing, but dead is dead. This GM doesn't view the the special ability twin souls as having a kind of limited omniscience, but another GM may. Such is a risky tack to take, however, possibly making a nigh-unkillable character, one that keeps his familiar in a secure yet comfortable lair miles (or continents or planes) away from any direct engagement, and who uses acquired bodies to depart the lair and harry his foe. This is, essentially, a dirt cheap (and likely murderous) variant of astral projection yet 7 levels early.
2 My gut says that with no receptacle should've said using the witch's or the familiar's body as a receptacle yet space constraints forbade that language. But that is totally speculation on my part and isn't what the ability says.
Best Answer
Unless the supernatural ability in question has a visible effect, such as causing claws to grow, breathing fire, creating an area of magical darkness or making the target larger. or turning him into a frog...
There is no visible or easy way to tell if your Evil Eye succeded
Supernatural abilities are very similar to spells, in that they are magical in nature, but they are not spells. So mundane methods to identify spells will not work against Su abilities.
For spells we got these rules that are clear:
But we got two paragraphs explaining how Supernatural Abilities work and are different from spells:
That meaning, the only visible way to tell if the target is being subject to a supernatural ability, is detecting their magical aura. But you would have to cast detect magic, use your evil eye, then see if their aura changed. Otherwise, the hex has no visible manifestation and is impossible to identify.
You can, however, identify that a witch has used her Evil Eye on someone (or on yourself), using a Knowledge(Arcana) check. The Spymaster Handbook Player Companion added some guidelines on how to identify class features being used.
This ruling is also supported by this answer.
Supernatural abilities can only be guessed by their effects, like the examples earlier, but what exactly they do should be unclear unless you somehow identified what causes the effect.
Such as when you see a person sucking the blood out of a villager, your first guess would be "a vampire!", but there are quite a few types of creatures that can do that, it could even be somebody with a weird taste of blood, and they all look humanoid and have similar abilities. In this case, you need to identify other abilities or features of those creatures (pointy ears? ugly face? aura of fear?).
Knowing the differences between Zombies and Ghouls can save your group from a total party kill.
For Evil Eye, there is no such visible effect. The description of the ability says:
So, there is nothing visible going on, the target simply has doubt on his mind and will suffer the penalties for your hex. The only thing you could do that will clearly tell you that he has doubt is to attempt to read his aura, using the Perception skill unlock from Occult Adventures.
You can also attempt to detect the thoughts of your target and see if he has doubt about the situation.
But do note that there is nothing that says detect doubt, so you will have to check with your GM before investing on it.
On the other hand, being so difficult to identify means you have the upper hand when attempting to remain undercover, the target will have doubts, but will not know exactly why. Meaning that you can apply Evil Eye on pretty much anyone that they will not know what you are doing (sans the knowledge check i mentioned before), as your Hex has no visible effects.