[RPG] Human Favored Class: Sorcerer

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.

I love the concept, Dr. Strange was one of my first super heroes to read a comic about. Would love to know how you intend to do the Eye of Agamotto.

1. I would suggest Warlock for first level; then dive into Sorcerer for a level for the AC boost. Then back to Warlock for another 2 levels. Finally, finish up with Sorcerer. So it winds up being WSWWS+. That first level will scream by pretty quick (one, maybe two sessions). You do delay 2nd level spell slots along with invocations by one level this way, so that's something to consider. Of the choices presented for order of classes, I'd say go with WWWS+. Using a feat to get eldritch blast when you know you'll get it for free in a level feels like a wasted feat usage to me. Plus it leaves that first level feat free for magic initiate (see answer to 3).

2. Dissonant Whispers I think is the better choice damage wise. It starts at 3d6 and will scale to 11d6 if you cast it using a 9th level sorcerer spell slot. I personally would say that outweighs the disadvantage that Vicious Mockery forces on a failed save. Plus, you can get this at first level, as Vicious Mockery can't be taken until 3rd level using your tome. Another option would be to get both spells, as the cantrip may have more opportunities for usage, being at-will versus limited by spell slots.

3. Take Magic Initiate as your first level feat and choose cleric as the class to get the cantrips, and first level spell from. Take sacred flame this way, gaining your radiant damage at first level. Also, you could pick shield of faith as your first level spell to be similar thematically to the shield discs Dr. Strange summons for protection. Another benefit of Shield of Faith is that it has no increase for higher spell slots, and the 1st level spell gained by MI will always be cast as if from a 1st level slot.