I have a CMMG upper that uses Colt pattern mags. I would say that you shouldn't overlook who makes the most reliable magwell adapter at the best price, in fact I'd say this is the key aspect to the 9mm upper.
I ended up having to get the Spikes Enhanced (aka, STEMB) to get my upper to run at my level of acceptance, and unfortunately that wasn't exactly cheap and pushed the overall cost well up. On the other hand, now that I have it, I love it.