There's about 2 grains difference in case capacity, in favor of the .260. Since both cases hold around 50 grains of powder with a bullet seated, a 4% difference, this means there's about a 1% "advantage" in potential muzzle velocity for the .260 with the same bullet weights, on average, when using the same powders and bullets, loaded to the same pressures.
Obviously different rifles with vary slightly, but if you feel 1% is enough to go .260 (meaning around 27 fps with 140-grain bullets) then you should also consider the fact that long, high BC boattails can be seated near the lands in a 6.5 Creedmoor with a standard 2.84" magazine box, but not in a .260. Thus accuracy will usually be somewhat better in a Creedmoor with high-BC bullets, which makes up the 1% difference in potential velocity in less than 75 yards.
A longer magazine will solve this in a .260, but overall the Creedmoor has so many advantages these days that unless you want to use cheap .308 brass to make .260 cases, there's no reason to choose the .260.
I am sure many .260 fans will argue this, but I have experience with the .260 going back almost 20 years, and plenty with the Creedmoor as well.
