Re: 260 or 308
I have owned 5 AR10's, 2 of them in .260 Remington, and I doubt I'll ever own a .308 again.
If you don't want to reload, +1 for the 6.5 Creedmoor. I've seen match 140 AMAX loads for $22 a box, and people are getting 1/2 MOA 4-5rd groups with that load.
I like .260 Remington, since I only hand-load for my precision guns, and there are so many brass sources for .260 Rem:
Lapua .260 Rem
necked-down Winchester 7mm-08
Nosler
Norma
Sako
Remington
If you choose the Creedmoor, and want to hand-load at a later date, you'll have plenty of brass to play with.