Everyone has made some great analysis of the firearms, and I believe that they are roughly equal quality. I think if you are getting a preban get a colt, if you are going postban get a bushmaster, other than that there is just the price difference.
Something else that you may want to consider is the dollar votes you are making in the politics of each company, too. Bushmaster has been under some severe scrutiny following the DC sniper fiasco, and they are strong supporters of gun rights, ownership, and make an active stand against anti-firearm legislation. Colt, on the otherhand, seems to neglect the rights of its civilian owners, given their large government contracts. As far as I have found they return none of their profits to the effort to ensure our freedom to own, or right bear arms. So, as you select a rifle, consider how you are spending your money, in more ways than value and quality.