AC2

Why not hard reset ? (but if u want to choose only between these 2, I choose AC2)

None. Go and buy something else.

AC2

How about we all make a forum topic asking what to buy/play eh?

AC2. Prototype is fun at first but later you do same things all over, and it becomes boring.

Between that games I prefer AC2

Both of them are the same "type". Both of them are good.
I would recommend AC2. Prototype is a little bit boring.

AC2

Witcher 2 ! (or AC2 )

Portal+Portal 2!

AC2 all the way

Skyrim!

Prototype is one of the worst games in the last 5 years. AC2 is solid and like 100000x better than protoype. But you could get Witcher 2 for the same price, which is better than AC2.

AC2, the best on it's franchise

AC2 all the way!! It's a fantastic game :D

Watch reviews, decide for yourself.