well that depends on what you're good at, what you like, and what you can get the best marks in. Theatre Criticism will require you to have very competent writing skills to get a good mark, and you should probably like writing, and be good at the review format. You need to be good at writing structured sophisticated pieces and at being concise. If that's you, then this may be your best option. Dont forget that it could end up being expensive for you, paying for tickets and transport to all the venues to see the productions, and keep in mind it would be good to see more than the minimum amount of productions (i think it's 4).
To do well in Performance it goes without saying that you need to be a good performer, that's probably the basic requirement. But you need to be motivated to persist with it (because you will have problems and have creative blocks), you have to be confident, willing to experiment and willing to read widely. It would helpt to have some sort of scriptwriting ability should you choose to compose your own piece, but even if you dont you will most likely have to adapt and shape the material you choose. So if that sounds more appealing go with that.
Ultimately it's up to you. Whatever you feel would be better for you overall in terms of ability, resources, etc. Good luck in choosing. Dont worry about it at this stage too much, you've got plenty of room to change your mind. Why not give them both a go?