Assassin's Creed
Pro's
Beautiful visuals
Incredibly expansive area available before you need a loading screen, I mean HUGE. Imagine like half the size of a campaign map on Halo.
Super badass fight moves and the occasional awesome line
Con's
Too easy to assassinate somebody and just walk away if they're not a main target or in a mission.
Gets repetetive quickly
Late-game you get attacked ridiculously easy
Mass Effect
Pro's
Solid plot, keeps you interested for a long time
Great chatting system
Interesting mix of RPG and Shooter
Con's
I'm no big fan of the combat
A couple positions are either really difficult and frustrating (Geth turrets near the end of the game 1-hit kill) or super easy and still frustrating (final battle)
You decide, but I prefer Assassin's Creed