In my humble opinion, I believe that bullet weight to bullet weight, the 6.5 is more ballistically superior over .308 - speaking in terms of bullets, not cartridges! That being said, I shoot a 6.5x55 Swede in a Remmy 700 and a .308 Win in a Ruger M77 both with 22" tubes. I shoot 140gr. A-Max @ 2865 fps. from the Swede and 155gr. A-Max @ 2910 fps. from the Winny.
6.5x55 140gr. A-Max .623 BC
100 yd. 2654 fps. 2189 ft. 0"
300 yd. 2375 fps. 1753 ft. -12.4"
500 yd. 2113 fps. 1388 ft. -49.6"
.308 Win. 155gr. A-Max .435 BC
100 yd. 2687 fps. 2485 ft. 0"
300 yd. 2290 fps. 1806 ft. -12.3"
500 yd. 1929 fps. 1280 ft. -51.4"
With the heavier bullet by 15 grains, the .308 Win holds it's own in the short race under 300 yards, but in the long race the 6.5 shines and carries the muzzle velocity and energy better with a lighter bullet. Granted it is not much of a difference and I do not think a deer shot through the boiler room is going to know the difference between the two, so pick your poison! Ammo availability and bullet selection definitely goes to the .308 Win, but all else would lead me to a 6.5.