Ok, I'm a Bushmaster fan. I've owned three, and all have been great.
That being said, DPMS is also a great company, and put out good products. Inlcuding their rifles.
BUT, the big thing here is what you are looking for in an AR15.
Do you want it for competition, and punching extremely small holes in paper?
Well then, I'd say that DPMS would be the best buy for the money. They make extremely accurate rifles, and at a fair price.
That being said, if you are wanting a Mil-Spec kind of AR15, than go Bushmaster. They have chrome lined chambers and barrels, as well as other Mil-Spec parts. Also, they chamber their rifles in 5.56 instead of .223. All a big plus in my opinion for a "military" style rifle.
Now I've heard that DPMS might be putting out chrome lined barrels, and chambered in 5.56 as of recent. If so, that would be a great step forward for them, as they are a great company. And I'd buy their stuff more if that was the case.
Anyway, hope this helps.