I will assume they are both post-ban rifles. IMHO, Colt is alot of name nowadays. Their preban stuff is grea, but the postban stuff is, well, kinda weak. Bushmaster however, is a great company that delivers great products. I own two Bushmasters and love every second I spend with them. The Bushmaster is a better weapon than the Colt in the post-ban versions, and cheaper as well.
Get a used Bushmaster off Gunbroker.com or on the Equiptment Exchange here. There are good deals out there.