I have experience with both the the 260 and 6.5 Creedmoor. Too bad Remington established the original twist rate of 1 in 10"for the 260. That mistake really killed the chance of a running start for the 260 out of the gate. Even though they changed the twist rate to a 1 in 9", quickly after realizing their error, this twist still prevented consistent stabilization of many bullets of over 130 grains in their factory rifles, especially the longer and more modern boat tail bullets that are all the rage today. Remington management didn't foresee the 260 being used with heavier projectiles possibly. It's use was purposed as a hunting cartridge, and with bullets up to 130 grains and their factory 1-9" twist rate it dies that satisfactorily.
Add this well known fact, and reinforce it with the ideas already spelled out in previous replies and posts, namely that even with the appropriate twist, the heavy bullet has to be seated so far in the case in a short action that the bullet consumes an appreciable amount of powder space. This renders the velocities achievable between the two cartridges of negligible difference.
Furthering the troubles with cartridges where a major portion of the bullet shank sits well past the base of the neck unsupported, articles have been being written speculating the impact of bullet deformation that occurs when the unsupported bullet section yaws in the case when entering the lead, essentially having the effect of pushing a limp noodle through a hole, to make an exaggerated analogy.
It has been speculated the minute deformation that can happen to the bullet, the deformation of which can be magnified by overly generous chamber dimensions, leads to a decrease in accuracy in said cartridges.
With all of that being said, I still shoot and like the 260 Remington. It's factory specs lend to less of a bullet jump to the lands when shooting lighter bullets. There is increased velocity with said bullets over the 6.5 CM due to the freed up space in the case. The shoulder angle of the 260 lends to easier feeding. And, as has always been the known, 260 brass can be made from any 308 based cartridge if desired.
The 6.5 CM was designed exclusively as a target cartridge for long range. It serves well in that purpose, and doubly well as a hunting cartridge with heavy bullets. The entirety of the long heavy bullets are much better supported to maintain alignment with the bore. It is my belief, that all machine work and components being equal, this lends to inherent accuracy going to the Creedmoor with heavy bullets.
The 6.5 Creedmoor, as has been stated, appears to be going into US Military duty, and if contracts for ammo production mirror that of the 7.62x51 and .556, it can be expected there will be an abundance of good military brass available to the public at some point.
So, in summary, do what you want, have fun and enjoy these great cartridges. I'll continue to shoot both or not.