I'll probably grab Metro 2033 when It's a lot cheaper. It's just SP right? If it has Bioshock 1 levels of awesomeness then it might be worth $50 just for SP alone, but for me It's hard to plop down $60 on a SP only FPS. It should have at the very least some throw away MP to extend It's bang for a buck.
Bioshock is still the only FPS on the 360 that I felt was worth $60 for the SP campaign alone. The second one isn't bad but It's more of the same without the epic plot twists.
It's just that simple.