6s and 6.5s are dominating the competition circuit for good reason. The projectiles are high BC, recoil is low, trajectory is flat, velocity is higher, and 6.5s are very efficient regardless of chamber. That said, I would check the shelves at local shops and see what they have available. If they carry factory CM, then buy the CM. If they carry 260, buy 260. If they don't carry any of em, then pick from a hat. I shoot a 260 but I would feel equally confident shooting a CM, x47, etc.
Since the performance is so similar, the decision comes down to components and ammunition for most guys. I can find 260 brass locally as well as 243 which is one operation to neck up (and saves a lot of cost) so it was an easy choice. I wouldn't get too caught up in the numbers, chances are if you saw 3 different 6.5s side by side you wouldn't be able to tell them apart.