Having both games I believe that I can offer a good opinion. Borderlands is an AMAZING game- however only I only recomned this if you have a friend also buying this game. Multiplayer is a blast, but the solo campaign is quite boring. Also this game is increddiably easy- final boss expecailly.
I just recently purchased Assasins Creed 2. Thinking the first was far to repetative I didnt have high hopes for AC2. I was wrong this game is great im about 20 hrs into it, and its sweet. So if your not planning on playing online this is the game for you.