FWIW...
I have a RPR in 6.5 CM, and a Howa 1500/KRG Bravo chassis in 308. Both have excellent glass.
Up to 600 yards, either is good and not too much of a difference in ballistics. 600-1200 yards the 6.5 shines, above 1200 you need something better. Price of ammo is similar if you watch for sales. Barrel life is not really an issue since the heavier 6.5 rounds (140-147 grain)do not burn barrels like the 120grain due to the lower velocity.
This sums it up
147 gr Hornady ELD-X 6.5 CM versus 178gr Hornady ELD-X in 308
drop/wind drift in inches with a 10 mph side wind
600 6.5 92"/19" 308 95"/25"
800 6.5 192"/38" 308 207"/48"
1000 6.5 346"/65" 308 386"/82"
1200 6.5 572"/103" 308 660"/129"
While the drop can be easily calculated, scopes will run out of elevation faster with the 308. Wind drift is huge, as getting the wind wrong will be be much more forgiving with the 6.5.
I took the MTC 5 day long range shooting course (highly recommended), and 3 of 5students and 1 instructor were shooting 6.5 and the other 2 students and the other instructor were shooting 308. At the end of the class, all 3 308 shooters said they will move to 6.5 after seeing the difference.
My .308 isnt going anywhere.
OP still hasnt answered how far he will be shooting. If its going to be under 500yds Id stay .308, cheaper more common ammo.
6.5 is the way to go.
308 and 30-06 get respect, but I think you can make the argument not enough respect.
They really are all most people even gun guys need or can make use of most of the time, most places.
But the ruger is not super expensive its not like he is buying a $4-5,000 rifle.
Lets just say I know a dozen guys with a dozen rifles and a lot of them are more than the ruger and they can't shoot for crap.
I say go for it, but do not expect the gun to make you a world beater or the rd to be a lazer beam.
It seems fun, I love the .308, and I want one of those rugers. I want the rim fire one as well.