Both caompanies make good quality materials.
Both have their little hang ups, mainly from the past, not recent years.
Armalites lifetime warrenty is what really makes the difference, when all is said and done, IMHO.
Armalite costs a little more. Bushmaster is cheaper.
Armalite has better fit/finish, but nothing really major, except for those who sleep with their rifles! :-)
Armalite has more calibers of interest.
For DCM, Bushmaster has not been having great luck recently. I know of many DCM rifles that have been problematic, from the past 2 years. BM service toldthem to shoot 1000 rounds and it will clear up...
Armalite can provide their super accurate stainless steel barrel.
When all is said and done, all of the top 5 or 10 companies pretty much produce the same thing. Many of the parts are produced by the same company(s).
Price and warrenty is where it all stands...