6.5 Creedmoor is the cartridge Remington wanted the .260 Rem to be, or so the theory goes. You can fascinate yourself for hours reading all the different opinions on the two.
It is only at longer ranges (lets say 500m+) that the Creedmoor starts to pull away from the .260 Rem due to the Creedmoor’s case design and ability to seat long for calibre bullets without sacrificing case capacity (30 deg vs 20 deg, etc etc). It’s at proper long range (1,000m) that the Creedmoor excels, which is what it was designed for from the ground up (whereas the .260 Rem is really only an adaptation of the .308).
We have started to use 6.5 Creedmoor because (a) we want to use the highest BC hunting bullets available in the 130-150gr weight range; (b) we like cheap, low recoiling, heavy barrelled rifles that are easy to shoot and pinpoint accurate at 400-600m+ without having to spend a fortune customising them; (c) we want readily available reloading components at sensible prices; (d) we have been sucked in hook, line and sinker by all the hype.
6.5 Creedmoor has become incredibly popular very quickly here, and supplies of rifles, ammunition and reloading components are easy to come by. We are shooting deer, goats and pigs at medium ranges and finding the terminal performance of the 6.5 Hornady pills to be comparable to any of the non-magnum 7mm or .308 pills. Lots of bang-flops at 400-600m, short distance staggers, etc. I have no qualms at all recommending the cartridge for medium game. In the last 2-3 weeks, two more 6.5 CMs have appeared amongst my hunting buddies.
Bottom line though is that at normal UK hunting ranges neither you, nor the deer, are going to know the difference between a .260 Rem or 6.5 Creedmoor. Buy either, you will kill deer, if you want to learn proper long range shooting, get the Creedmoor.