one is mission oriented
another is open-ended ala GTA/Infamous/Prototype style

I would say AC2 is abit more value for money, U2 can easily tapao within 15 hours for the single player mode, though its replay value is quite high as it can unlock additional stuffs in 2nd play

AC2 got a bit RPG element, it has more freedom

actually just buy both lo, both also got pretty decent resale value, no worries no one want ur used game later