If you are building a DMR, the M14 is a better base because it has a better hop up unit (AUG), stronger gearbox (V7), and everyone and their mom has an armalite.
The G&G and CA M14 gearboxes do not use ANY V7 parts, and are NOT v7 gearboxes. The V7 gearbox is much weaker than the G&G and CA M14 gearboxes...
Having said that, G&G offers a version of their M14 that chronos at ~430 w/ .20's. It's called the G&G M14 Veteran, and is about 360$ for a full metal full wood M14 with a bullet proof gearbox. It is literally a hop up rubber and nub away from being a tack driver.
As a good suggestion, replace the piston head with a #14 O-ring. Mine now chronos at 390 w/ .25's up from 383 w/ .25's stock. I can reach 80 yards (Laser range found) and be 50% confident I'll hit what I'm aiming at with .25's. I've got some .28's that I'm going to try and push closer to 100 yards of range, and maybe not get there, but thats ok