I don't recall off the top of my head but I want to say the big difference between the two is the transonic range, e.g., the distance where the projectile moves from supersonic to subsonic.
IIRC for the weight you suggest, .308 Win transitions to subsonic somewhere between 800-900 yards, while the 7mm-08 is still cooking supersonic at around 1100 yards. The transition from super- to subsonic imparts an instability on the projectile which affects accuracy.
That being said, a 6.5mm/.264 projectile stomps azz on both the .30 and 7mm/.284 calibers for retaining velocity and accuracy downrange.
Now, all that being said, if you are looking at an 18" barrel just stick with .308 Win IMO. Plenty of ammo choices at a cheaper price, with less wear on your barrel, and plenty of "in-stock" components.