For those with reasonable knowledge of both, is there an advantage of one over the other? The Creed is no doubt popular, but the 260 has more case capacity.
Why isn't the 260 the match winner that the Creed is?
There are some subtle but important differences between the two. The 6.5 Creedmoor has a shorter case and a longer neck, so bullets can be seated farther out of the case at the same overall length as the 260 Remington. The bullets don't intrude into the powder space of the cartridge, and the longer neck provides better support and alignment of the bullet. To gain back some powder capacity, the 6.5 CM has less body taper and a shoulder angle increased from 20 degrees to 30 degrees. The less tapered case aligns better in sizing dies, possibly improving the neck sizing operation. The 30 degree shoulder angle is in line with many successful target cartridges. The standard twist rate for the 6.5 CM is 1 in 8 vs 1 in 9 for the 260 Rem, so it better stabilizes the longer bullets that are popular in the long range game. Essentially the 6.5 CM is an improved version of the 260 Rem, using many of the design details from the PPC series of target cartridges.
What's the difference, about $100-$300 depending on the rifle purchased (and your shopping ability). I went with the .260 because I got my stainless laminated Tikka for close to $300 cheaper than the going price for the same model in 6.5 CM.
If you reload and you find a better deal on the rifle, the .260 is a great choice.
If you don't reload or intend to use heavier bullets for target shooting, then the 6.5 is a better choice.