Well, they're both great games. Here are some pros and cons to help you decide.
BIOSHOCK 2
Pros:
-Multiplayer
- Gameplay mechanism
- character models
- Story.
Cons
- Textures (they're sometimes washed out and not as detailed as the first)
- No backtracking / Too linear
- Audio recordings not as good as BS1 (Ryan's voice changed)
- Lack of 'sound charm' from the first one (less 50's music, vending machine sounds etc)
Mass Effect 2
pros:
level design
Mordin and his singing xD
Space hamster!
Improved gameplay from first ME
Recruitable characters missions
Mini-Games
Cons:
Puzzles could have more meaning than just to get some credits, or rarely equip upgrades
It gets repetitive
Easy
One command for use/cover/sprint -_-
Still very linear
No shortucts for weapons