Both have pluses and minuses (very minor ones) that make them pretty equal when comparing the two rifles side-by-side. But some Colt models have more pluses than others. Let's compare full-featured, LEO M4s, which gives the most "pluses" to Colt:
Colt
Pluses:
M4 handguards, side-sling swivel, extended feedramps, 1:7 twist all standard. Some will have T-marked uppers. Better assembly QC.
Minuses:
Non-standard-size fire control parts, closed "bridge" sear block, very low availability, very high cost, poor service, questionable RKBA supporters.
Bushmaster
Pluses:
All Mil-Spec part sizes, standard sear block (accepts DIASs), 1:9 or 1:7 barrels available, good availability, lower cost. Very pro-RKBA company.
Minuses:
M4 handguards, side-sling extra. T-numbers and extended feedramps not available (ramps can be added "aftermarket" though). Barrel-to-upper alignment problems on some rifles.
Now, many civilian Colt models have additional minuses, such as: plastic buffers, no chrome bores, plastic buttplate doors, screws instead of pivot push-pin. If you are comparing guns that don't use military barrel profiles, these minues probably apply.
-Troy