Not certain what type of piston the CMMG uses...but, if it's a short stroke, carrier tilt issues have been rearing up in the piston AR's out there.
Personally, I have no issues with DGI rifles; if you clean it fairly regularly, and run the bolt wet, you shouldn't ever really have a problem. I've got a couple rifles that I've put over 500 rounds through in an afternoon, without a single problem...
I guess, to me, piston drives make sense in SBR'd and full auto AR's...
ETA--as to the which and why...
If you like the features of the Wasp, go with that; but, then again, if you are sold on piston drive, you can always add features you like (such as the rail, etc) to it later.
Guess I'd get the Wasp, though.