Why the Creed, because Remington failed, they are very good at cartridge introductions and bad at offering the right rifles for the rounds, proper twists, proper ammo, etc. & to cap it off do a lousy marketing job.The 6mm is a good example, by starting with a slow twist, only light bullets, the 243 kicked butt (and yes it's good in it's own right), ah 6.5 & .350 Rem Mag, SAUM's , we could go on and on.
Even though the 260 may not be the absolute perfect short action std. casehead 6.5, it is a great round and if Remington AGAIN had offered all the rifle versions wanted and a little better ammo, faster twists and good marketing the 6.5 Creedmoor likely would have never happened. You could argue that the 300WM is less than perfect (short neck, belted) but Winchester did not make those mistakes mentioned above but it is doubtful any 30 cal mag no matter how well designed will pass it by.
And so the Creedmoor is passing the 260 by, if you don't believe it look at how many newer rifles chamber the Creedmoor and ignore the 260. Also, look at new ammo offerings in the last 2-3 years. But if you have the 260 brass who cares or you can form it from other 308 based based rounds if you wish.
To me it's simple, if you have a 260 and like it, there is no need to get a Creedmoor. If you have a Creedmoor and like it there is no reason, zippo for getting a 260.
The 260 has slightly more powder capacity than the Creedmoor when you look at the case itself and if you shoot lighter bullets then you will have a little more velocity.Most people who buy the Creedmoor like the ballistic advantages (whether they can utilize it or not) of the sleeker bullets. The Creedmoor is a little shorter but with a straighter taper and the design allows you to seat the longer bullets out further & thereby the velocity with those bullets is on par with the 260 & is a little better design.
If someone is starting from scratch I would have to advise them to go with a Creedmoor, the advantages outweigh the disadvantages by far. This link does a great job explaining the 2 case designs and the advantages of the Creedmoor.