The whole colt having Proprietary pins is bull, the original colt rifles from Vietnam area had different pins but you cant buy anything like that any more, it only comes in to play if you have old old colt receiver.
Colt makes a better rifle then bushmaster, thats not to say that bushmaster is not a great gun or suitable for most every use.
Colt Is mil-spec, bushmaster is not......bushmaster may build to mil-spec but they don't mag particle check every barrel after having shot a proof round as colt dose, bushmaster only check lots of barrels 1 out of ever how many.
Colt actually has an out side inspector that checks the rifle to insure they are mil-spec, bushmaster dose not.
Colt is available in 1x7 twist where bushmaster is available only in 1x9 twist, this is the only reason I would choose the colt over the bushmaster. The 1x7 twist will allow accurate use of the MK262MOD1 ammo available now.
If It wasn't colt I would save even more money and buy a Double star or stag.
Here is the run down of the AR15 read this