Since you asked specifically about .308 vs 6.5cm, I'll keep it on topic here.
I ran a bench .308 for about 12 years before finally swapping to a 6.5cm. So I have fairly extensive experience with the .308, the 6.5 is still new to me. At first glance I can tell you this. 6.5 simply outperforms the .308 especially if you're looking at factory ammo.
Looking at standard factory match ammo for each - 168gr federal GMM for 308 and Hornady 143gr eldx for the 6.5.
The 308 has more energy to 350 yards, at which point the 6.5 outruns it. At 1000, the 6.5 has more than 9 feet LESS drop than the .308, to say nothing of wind deflection.
Folks like to hate on 6.5 because it's new, and a lot of people here are happy old dinosaurs who don't like change. But 6.5 flat works. Nothing wrong with a 308, and I have shot out to a mile with mine (although it really dies hard around 1100 yards) but a 6.5cm will reliably get you to a mile with half the adjustment of a .308.
These two ballistic charts are from strelok, using factory ammo specs and factory velocities (which I've found to be fairly close to real world). The .308 chart is the one that's red on the bottom.
For barrel contours, with in reason a bench rifle will always benefit from a heavier barrel and heavier overall setup. Most dedicated bench rifles range from 14-20 pounds, and some get dramatically heavier. Mine runs about 13 pounds, and a buddy's is 15.5. You can feel the difference in recoil between the two guns. Heavier means you'll resist shakes more, you'll see less recoil, stay on target after the shot easier, watch impacts easier. A heavier rifle is also less susceptible to a poor trigger press pushing it side to side. The specific barrel contour doesn't matter that much in the grand scheme of things, although I'm sure some benchrest guys know far more than I do and have info on how specific contours can affect accuracy. For what you're doing, it won't matter. get a heavy, 22-24" bull barrel 6.5 cm in a chassis with a clean trigger (trigger tech diamond if you can swing it) and you'll be golden.