Headhunter,
I'm no expert and frankly wished I'd looked further into the .264 caliber before building one, but I have a 6.5 X 55 being finished up. As you may know both the 260 and 6.5 Creedmoor are the same caliber. The difference is the size of their casings and both high a high BC and are very efficient and highly accurate long range rounds. I guess you would have to look into what purpose you intend to use it for, other than "shooting long rang." If I had to do it all over again, since I shoot for accuracy and not hunting, personally I'd choose the 260 which is the one of the two you mentioned that has been around the longest. The Creedmoor is a much newer invention that many target shooters think is the ticket in accuracy. Also, if I remember correctly, the 260 has the smaller casing of the two and has the velocity similar to the 6.5 X 55 Mauser. The one thing I'm sure of is that the 6.5 is extremely accurate, especially out to 1K and I think either one would work fine for you. But it's the purpose that should matter for you. And I'd also be interested for see how well that rig shoots using that Savage Model 12 action. The reason I say that is because unless you change out the trigger mechanism (with that darn "accutrigger- sear block foolishness), depth seating may not work up to the rifle's accuracy potential when you try and jam the bullet say .030 into the lands. That's what drives me NUTS in all my Savages and even the one Model 12 that has the target trigger mechanism. I've shot a couple of 260's having the Remington Action and one with a Shilen and the other with a Bartlein barrel and they made me look like I could really shoot. Good luck in your build and hope she performs to your expectations. I know the ones I've shot, shoot better than I anticipated using Berger 140 Target Bullets and especially with Sierra 142 HPBT Target's.