The idea is 3 Knife Master / 1 White-haired witch / 1 Sorcerer [with a bloodline that grants many free feats* which synergize with a rogue], then X Knife Master & Scout. Originally the plan was 3 KM/ 2 Umbral Weaver/3 Shadowdancer/X KM, just to get a cool pet, so I'm not too worried about optimizing, it just has to be useful/usable.
The reason I'm doing it this way, because I like playing rogue, but the party (ranger, druid, cleric/dwarf, cavalier) has no arcane caster, so I want to add the flexibility of using effective (3rd CL) first level spells. It should work good with combat/flanking.
More info: STR 10, DEX 20, CON 14, INT 16, WIS 12, CHA 16 [though if you can tell me a good reason to, for the next 6 h real time these could still be changed].
Where do you think the trait's effect would be more powerful and why?
Update: Of course that question heavily depends on which spells are used, so more info:
Sorcerer-spells: know 4x0th, 2x1st; can cast 4x1st spells.
Witch-spells: known all 0th, and 1st (though from a different list); can prepare 4x0th and 2x1st per day.
The Witch 0th spells mostly don't care too much for levels: guidance, resistance, light(*), daze, mending(*), Arcane mark, detect magic (*), read magic(*). Some (*) are affected by it (mostly duration), but since they're cantrips I could just cast again.
At 1st some aren't too significantly affected (Rejuvenate familiar(*), blood money, command, Beguiling Gift, sow thoughts, charm (* – can just use diplomacy to make it a real friend), hypnotism, etc.) – however the DC depends on her caster level, so many spells simply might not hit with just a 1 lvl dip without the Magical Knack bonus.
The Sorcerer 0th Ghost Sound and Haunted Fey Aspect would have to last more rounds so I can create them, do something else, maintain (or recast) them, do something else, etc. I looked at a variety of 1st levels and have a hard time choosing. Summon Monster, Expeditious Retreat (3 min should last the battle, most often), Protection from Good/Evil/Law, and a few others such as animate rope, reduce person (bonus to Dex, and allows for riding a medium sized creature into battle,…).
For the moment assume that either is just a 1 level dip.
*[here's an example of a cool bloodline: Dreamspun for skills [+2 (and +4 after 10HD) to Bluff, Diplomacy and Perception, +2 Intimidate and Disguise, +5 (maybe +10 at lvl 10) Sense Motive], and pre-requirements [Blind-Fight, Combat Expertise, Improved Feint – that and having spells should be worth losing 6 skill points, up to 2 HP and 3/4 bab for, right? – there's many other, some even cooler bloodlines.]
EDIT/UPDATE: Turns out the Sorcerer's feats aren't by any means free. Because I didn't intend to stay I didn't look further ahead into the progression, had I done that I'd have noticed that you get to select from that list of feats, way way into the class. You don't just get all of them at lvl 1! That explains a lot! 😀 – I was confused how that could possibly be balanced to get say 3 useful feats and a dozen skill points for 1 lvl dip.
Best Answer
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.