Unless you are target or varmint shooter who will burn out a barrel quickly, the choice is determined by how big a critter you pln to shoot with it. If nothing larger than a deer, then the 6mm will handle big enough bullets for the job. But if you plan to go to shooting elk, etc., then get the 6.5 so you can shoot 140 and 160-grain bullets when required.

I have 6mm/284. But would probably like it better IF it was a 6.5mm.