Really hard to say when you give no info on what you are going to be doing with it--

6.5 PRC has obvious ballistic advantages over the 6.5 CM.... but at a cost- so what's the need? If it's just a want to try something different then I'd go with a 6mm (but with a BR or BR variant).

I had a 6CM that I was pushing hard- and I didn't make it to 800 rounds before the barrel was done- might have been less than that but it's been a few years. 6mm's seem to dominate prs stuff- PRC is pretty popular with hunters (and some target shooters) and when going for small groups at longer range most people use a 7mm or 30 cal.