I put together an Aero Precision EPC. In hindsight, I'm not sure why I bothered as blowback isn't really that enjoyable after shooting better operating systems.

I'd suggest you really consider the Sig MPX. It's a duty and/or competition quality firearm with some pretty innovative engineering. It's a good weapon and one of my favorites. If you've got an MP5, you'll appreciate the smooth cycling of the short stroke gas piston. I actually think it's a bit smoother than the roller lock design.

If that just doesn't do it for you, then the CMMG radial delayed blowback would probably be the next thing I'd consider.

After that, choosing a blowback AR9s would be a function of personal preference for the features / brand you like, but kind of like a 308 AR I'd suggest trying to keep as many of the parts as possible from the same manufacturer to help ensure it works well out the box, but I generally view all of them as range toys. My EPC is fun, I guess, but I'm about to send the barrel back to AP so they can replace it with one with an improved feed ramp design as they are known to have issues with flat nose / TMJ rounds.