Gears 2 or NFS?

These are the two games I might get from games on demand but i don't know which to get

I want to try gears 2 again due to the fixes of some of the problems i hated about it (smoke takedown and shotgun kill)

But i need some variety NFS Undercover or possibly Mass Effect

What are some of the best things about these games?

This would be better suited to be posted in the game recommendation sticky :)