If I was going to build one, it would be in 260 Remington, because I already have some 260 Rem components. However, the 6.5 Creedmoor has gained enough popularity and most especially AVAILABILITY that I would recommend someone else building on go with that cartridge.
Due to the great ballistics of the full size rifle cartridges in 6.5mm (260 Rem, 6.5 Creedmoor, 6.5x47 and in long action the 6.5x55) they begin to surpass the energy of the 308 loads at around 300-400 yards, and from then until going transonic, retain more energy than the 308 rounds.
If someone has an immense quantity of 308 ammo on hand, or has special access to 308 ammo (such as military access) then I would go with 308, but otherwise the 6.5 mm cartridges I mentioned have less recoil, better ballistics, better wind resistance, longer range, and from 300-400 yards, more energy than the 308.