Actually, we don't use DirectX. Just XInput (the X360 Controller drivers). Sorry if something I said somewhere was misleading.

Right now we use SDL for input and window management, and OpenGL for rendering. The main reason for that is just 'cause I prefer the way OpenGL is set up. I prefer SDL to Allegro, just 'cause I think Allegro has a lot of components I wouldn't use in this situation. (although I love Allegro and used to use it back in the old days!)
DirectXPro: Supported on Windows XP and Vista, makes XBox360 port easier
Con: Not supported by Mac.
OpenGLPro: Supported on Mac
Con: Not supported at all or very well on Vista yet.

(don't have too many details on this, but hopefully it'll change)