Do you hand load? If so, the x47 case is really good, very nicely optimized. There's no real answer to what's best. It's what's best for your use case that's at question.
If not .260rem is a hell of a lot more available as is 6.5CM. They're all pretty much in the same basic performance boat, only small differences between them. Of note, a .260AI is basically a match for the 6.5PRC with the proper powder selection except it uses a .473" bolt face. A buddy of mine shoots a .260AI in competition. The thing is hard on targets. 2900fps+ with 140gr class bullets.
The 6.5PRC just doesn't seem like it's a great option for anything. 200fps over a 6.5CM for 50% more powder burned just makes a ton of extra recoil and blast for what is ballistically nothing. And by nothing I mean it. The difference in drop at 500yrds is about 4 inches. It's a great way to put to use a magnum bolt face short action that was built on something abusive in recoil like a .325WSM but IMO, that's all. It seems made to comply with match rules and to give just a little velocity edge to make for less wind drift variance for match shooters.
