Originally Posted by glenn asher
The .260 is a necked-down .308 case, simple enough. The 6.5 Creedmoor is a .250 AI case necked up to, errrrr, 6.5. It's based on the shorter .300 Savage (or .250 Savage, or .22/250 case, necked up and blown out). Not much difference, all told. Maybe a couple of grains difference in case capacity, that's about it.
I think that answers the OP's original question better than any of the other answers/comments I've read so far.
An even better and more factual answer is to say that the 6.5 CM was actually developed from the 30 TC cartridge necked down. Both the 30 TC and the 6.5 CM are Hornady creations from the 2007-08 era. Granted, other people have successfully "made" 6.5 CM cases from other more familiar cartridges, but its actual origin is with the 30 TC case. As Hornday states in their 9th Edition manual, the 6.5 CM is "Based on the 30 TC, a 2007 Hornady introduction, it is a shortened and improved design based loosely on the 308 Winchester".
With readily available and reasonably priced 6.5CM cases and loaded ammunition, it makes sense to just buy either of them unless you need something extra to do on cold winter nights.