What are your inputs on these two piston systems?
Thanks in advance!

The AA is a "pull through" piston design (rod comes out through the front of the gas block), leaving nothing behind. The CMMG is also a "pull through", but leaves behind a spring tube which is attached to the gas block.
They both work similarly, but have slight differences. An AA requires a mil spec barrel nut due to it's piston rod spring cup (has to rest against the barrel nut). The CMMG does not require a mil spec barrel nut is more friendly with FF rails since it's piston rod spring is captured at the gas block end (that tube that's attached to the gas block). However, that spring tube may require opening up some FF rails ID's at the front end a bit.
The AA uses a cam type setup to retain it's "plug", where you just twist an pull out. The CMMG uses an "end cap" (a threaded screw/bolt that must be removed prior to pulling through).
They both use proprietary BCG's with everything else mil spec.
AA is MUCH more prevalent in the aftermarket sector, where as the CMMG is less so but is used (versions of) in factory rifles (Ruger, Stag both use CMMG variants). CMMG seems more interested in the factory level sales, and AA seems focused on the aftermarket level sales.
Both seem like great systems and you'd not go wrong with either one.

I've owned both. The CMMG I had was more accurate than the AA with XM193 ammo. Both had 1/7 twist barrels, so shooting 55 gr ammo may not have been the best match for the barrel. It's easier to adjust the gas settings and take down the AA system. It's difficult to adjust or remove the threaded plug on the CMMG system by hand due to carbon build-up even after minimal rounds fired. I favored the AA system because the gas block was also lighter in design. I have since moved on to an LWRC rifle, which I would recommend over both CMMG and AA.

I own LWRCi rifles now, but I've had Adams Arms (EVO and Tactical Elite) and a CMMG piston.  To be honest, my CMMG ran like a champ, but I too didn't like the way I had to unscrew the bolt to get to the piston components.  The Adams Arms is a breeze to break down and it just seems less "gaudy" in its manufacturing.  If I had to make the choice again, I would choose the AA.  However, don't discount the CMMG (Stag Arms) piston gun...especially if you can get one at a good price.  Good luck!!