260 Rem and 6.5mm Hornady Creedmoor are ballistically very close. Design wise, both use the 308 Win / .30-06 case-head lower case-body set up and are equally strong, SAAMI setting an MAP of 60,190 psi for both (same as 308 Win). Both have the same internal case capacity of ~53.5gn water. Both have 2.800" as their nominal COALs, and both are capable of superb precision in a good rifle.
Personal whim aside, the choice comes down to case design / brass. 260 Rem is an adaptation of a 60 year old .30 calibre cartridge originally designed to handle relatively short 144-150gn bullets in that calibre. A 140gn 6.5mm bullet is much longer and is therefore seated very deep in the case if the 2.800" COAL has to be adhered to (magazine use and/or barrel throating). In theory this puts it in a sub-optimal position as well as reducing case capacity and space for powder significantly. The Creedmoor is a more modern design optimised for the 6.5mm calibre and still designed for good magazine feeding in short action rifles and at 2.800 COALs. It uses a slightly shorter case with less body taper and a sharper (30-deg) shoulder angle. It therefore (in theory) handles 140gn bullets better.
Both designs have no trouble at all with 120/123gn match bullets and generally little with 130s, it's all about 140s.
In practice as opposed to theory, the 260 seems to manage just fine with 140s usually shooting them very well and not losing much on velocity. However, the Creedmoor is an equally fine performer, so it remains a coin toss job.
The other factor is brass supply/ quality. With 260 getting on for 20 years old as a factory number, most ammo companies now make brass for it, Winchester and Lapua joining Remington, Federal, Nosler and Norma fairly recently. So there is a good choice, and the Lapua stuff is very good indeed if pricey. The Hornady design is now where the 260 was 15 years ago - a near new model with brass provided by its designers (Hornady) and everybody else looking at it to see if it's proving a 2-year wonder or joining the mainstream as a solid seller. The signs are it's the latter with strong sales, continuing interest, other toolmaking companies joining Hornady in producing dies etc. It was reported on the Daily Bulletin for this year's SHOT announcements that Winchester was going to start loading match ammo, in which case they would make cases too obviously, but there is still no mention of either on the Winchester website and I've seen no references to this either. Norma has reputedly added the Creedmoor to its range too, but again I've not heard of any of its brass in use.
There were a few reported quality issues about Hornady Creedmoor brass quality in the early days, but these have obviously been sorted (if they really existed anyway). Supply was problematic early on too with periodic spells of unavailability especially as unloaded brass, but this seems to have eased too. (I can't comment accurately on this being in the UK and the cartridge only marginally used here.) I think we will see the Creedmoor eventually provided by a range of suppliers, but it's been a bad time to adopt a new design with most companies working at 100% output on existing models to keep up with demand.
I have rifles in both cartridges and like both pretty well equally. The Savage LRP looks a very interesting rifle and I have one on order in 6.5 Creedmoor but the UK importer is waiting on Savage sending an example, it proving difficult to prise any out of the US market especially in Creedmoor. I suspect (have yet to prove) that Creedmoor will handle 140gn class bullets a little better than the 260 or 6.5X47 Lapua in which case it'll likely be a bit better at 800 - 1,000 yards than the other two where I load little other than Lapua 123gn Scenars
