[RPG] How many enemies can one Lvl 1 PC handle

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.

Problem one is that the CR2 means it's meant for 4 2nd level characters (defined as "they should use 20% of their resources on it"). You're already going to be shorthanded by a fair bit. (Two level-2 characters are ECL 1 - the equivalent of four first-levels).

The ratling and the dire rats together are a CR 3 encounter, so let's split them up:

Send in the dire rats early. They're a CR1, so just right for your two guys. If it works out to be a fair fight, they can take a moment to rest before the ratling makes his appearance, which progresses to a CR2 "boss fight".

If they're doing really (read: too) well against the dire rats, you can always pull the ratling in early to push the challenge up a bit. But this way you have some options.

An alternative is to set up the ratling as a bit more of a "boss", letting the dire rats do the fighting, and then making a Dr. Claw style retreat ("I'll get you next time... NEXT TIME!") as the conclusion of the encounter. (And again, you can scale his participation to keep things interesting.)