I don't have a DPMS, so I can't comment about them in terms of quality, cost, etc. I do have an Armalite AR10 though.
Quality - Armalite has been top notch. I do have some cycling problems to deal with, but I created them. I cut down a 100% reliable 24" T barrel to 18" and got some cycling problems. I expected that though, and there is no way Armalite can be held accountable for that.
Price - I think these two rifles are probably as close to the same as I can find:
Armalite AR10A4 Carbine - MSRP is $1,506 ($1,299 at Impact Guns)
DPMS 308 AP4 Carbine - MSRP is $1,249. ($1,199 at Impact Guns)
So while the MSRP is hundreds of dollars different, street price is only $100 different. Not much in my opinion. If you buy the Armalite from Quantico Arms, it is $1,203, so the difference is $4!. Street price on DPMS mags seems to be about $30 to $35, while Armalite's are $35 to $40. Also, I think there are far more "custom" parts available for the AR10 versus the DPMS. I think the aftermarket community is catching up with DPMS, but I think Armalite still has the edge.
The DPMS is brand new, basically an unproven platform. The Armalite in its current configuration has been around since 1996. I think many of the bugs have been worked out of it. Also, the Armalite's can be had with chrome lined barrels, I don't know about the DPMS.
That isn't to say that all AR10s work 100% right out of the box (like they should.) I'm not certain that can all be attributed to the rifle though, as many people use cheap surplus ammo in their rifles (SA 308 is from the early 80's I believe, over 20 years old!)
I personally would take the Armalite unless there was a significant savings to the DPMS (and all the parts I wanted were available for it, such as Larue rails and gas blocks, etc.) I think the DPMS is probably a good weapon though.
And, in true ARFCOM fashion....
GET BOTH!