In my opinion the M14 makes a much better looking DMR. The AK is a million times easier to work on than any M14 design, but both will require the same type of work to be a viable DMR and outshoot a standard upgraded airsoft gun.
Building a DMR isn't just sticking in a TBB and new hop up rubber, especially with a CYMA gun. Their lousy part fitting, loose tolerances and very poor air seal require more fixing and tuning than a higher quality brand. Everyone changes their hop up and runs aftermarket barrels anyway, so you won't have any advantage whatsoever. You should expect to spend several hundred dollars in quality upgrade parts to build a viable DMR that has noticeably more effective range and even then the extended range over a standard upgraded airsoft gun is not incredible.
I went M14, and I will never regret it. My wallet might.