TBH, BIA was a VERY big disapointment, IMO. I don't know why, I just don't think it was as good as it was hyped up to be. Definitely get SC: Chaos Theory, loved the SP series and it keeps getting better and better with every game.
depends what you're more into. Both are/will be good games, if you like Army/shooter get BiA, if you like stealth get CT.