Even over just the last year there has been an explosion in the 9mm AR market. Instead of having like two people making mag blocks and dedicated lowers being unheard of, we have options for pretty much anything you'd want. For Glock mags, Palmetto State Armory, QC10, Joe Bob Outfitters, and Lone Wolf are the go-tos. Hell, even Wilson Combat is making dedicated Glock mag lowers. For Colt, you can either go the mag block route if you want to make it on an existing lower, or the dedicated route if you want something new. Same names apply here, but don't forget the mag block manufacturers like Hahn.
They're both going to be equal in the reliability department since they both use a proven magazine design and have a really simple action. I have a stack of Glock magazines, and there are cheaper and cheaper options coming on the market everyday (ETS anyone?), so that'd be my choice. But my brother has a Colt mag lower, and I saw a guy running a Colt mag lower without issue in PCC at a USPSA match last weekend.
Most of the dedicated lowers have bolt open, while the mag block lowers are hit or miss. Just make sure you know what you're getting before you purchase.
That said, while I'm going to buy a 9mm carbine soon, it's not going to be an AR. I'm 90% settled on getting a Sig MPX and having Morgan do one of his K conversions to it. As a lefty, these new ambidextrous designs are really attractive. Since I'm also going to file a Form 1 for it, having something that has a truly collapsible stock instead of just a retractable one is something I'm drawn to. Stacks of Glock mags be damned, I want a gun that I can carry in a briefcacse and feel like James Bond.