Ive had 2 buddies now that started on .308 and regret it. Usually the logic behind 308 is "The military uses it" or "ill be able to find ammo/components for it everywhere" and "its a time-proven cartridge".
Then they find out that when it comes to match ammo, (not cheap plinking ammo) there is just as much 6.5 Creed ammo and components on the shelves as .308.
Only advantage to .308 is it has nearly 2x the estimated barrel life. But to be honest, most of us will take several years to shoot out a barrel (if ever), and when/if we do, we can always get a new caliber, contour, or length. The barrel is a consumable, and isnt that expensive in the grand scheme of things. A few hundred bucks for a pre-fit barrel that lasts several thousands of dollars worth of ammo. If you get a .308 barrel (which lasts quite a deal longer) youre kinda locked in to an inferior caliber while shooters to your left and right will be shooting 6 and 6.5mm.
BUT if 600 yards is the furthest you intend to go, .308 will do just fine. I start to struggle with .308 at around the 800 yard mark.
My vote is easily 6.5 CM. Its the logical choice unless you have some sort of personal reason to go with .308.
As for the gun. If you like the chassis the RPR comes in, and you dont mind it's tank-like handling, then Id go for that one. Be sure to try handling one at a store if you can.
If you think you may want to change out the stock, go Howa or Tikka.
The RPR cannot be taken out of its chassis, you can swap some parts on it but you are stuck with the chassis. Not that its a bad chassis, just permanent.