Quite frankly, the key advantage to 308 is the barrel lasts longer. My savage 308 got 6500 rounds through before shitting the bed, but for excellent accuracy, you should expect around half that with a 6.5.
Otherwise, the 6.5 is better in nearly every way. At 500-600 yards there won't be a *lot* of difference, but past ~600 it's night and day in terms of ballistics. On a mild condition day with a light 3-4mph wind that's a bit switchy, it's damn easy to miss a 1moa target at 900-1000 yards a LOT with a 308. Just left, just right, just left, just right... With a 6.5, it's hit, hit, hit, hit.
Some people might say the missing with 308 like that is good "practice". I call it "frustrating". ANY cartridge will require you to dope the wind @ 1000 yards, or even 600. So you won't be cheating yourself out of some kind of fundamental learning by skipping 308. Regardless, to make good hits, you'll need to judge wind and account for it. At least with a 6.5, you'll score some good hits in the process.
Now, comparing all the 6.5mm cartridges out there, the key advantage of the Creedmoor is good factory ammo for reasonable money. Figure ~$27 for 20 rounds. 260 will be much more. If you reload, its kind of a non issue.
Comparing ballistics between the common 6.5mm cartridges is an exercise in futility. They're all basically the same.
Last point, because I know you're thinking about it, as an amateur... the thought of wearing out a barrel and needing a new one sooner might be daunting. Maybe you think its expensive. It's neither. It's a DIY affair for ~$350 on a savage or ~$700 on other rifles for a premium barrel installed by a premium gunsmith. When you figure cost per round in barrel consumption for a ~3500 vs 7000 round barrel, yyou'll see its only pennies per shot.
