If your happy with the 308 round, and you shoot 600yds, I see no reason to go 6.5. The 308 barrel will outlast the 6.5 by year's. Shoot what you are comfortable with.
Two facts I should have mentioned: I only use commercial ammo (lack time/dedication to reload) and I'm not shooting enough that barrel life is a huge issue. I'm also looking only at actions that have prefit barrel options, so future replacement should by DIY and just the $400-500 cost of the barrel.
With that said, I don't think I'm happy with .308 any more. Recoil isn't that high but it gets tedious after 60+ shots. I've been using FGMM, 168 in my 1/12 rifle and 175 in my 1/10. I won't claim to be super skilled but variable wind is my biggest source of error right now at 600 yards known distance. I'm scoring as well with my .308 as many of the local competitors who are shooting flatter/higher BC rounds, which makes me think that the .308 ballistics are holding me back... and because F-TR is not treated like a separate class here, I don't get to win my class, I just finish midpack among the overall group.
After looking around I identified the flattest, least wind-drifting .308 factory ammo as Hornady Superformance Match ELD, with a 168gr .523 BC at 2840fps. Per two ballistic calculators it does pretty well at 600, but various 6.5 CM loads do equally well with slightly lower MV but higher BC, and a few of them have noticeably less wind drift, the Hornady 147 ELD in particular. I was about to order a few cases of the Hornady Superformance Match ELD 168 and stay with 308, but I don't have any yet, and I'm looking at buying a new rifle anyway. With that ammo at 1.40/rd and various match grade 6.5 CM at 1.20-1.30/rd, I'm questioning the logic of sticking with a more drifting round that has more recoil and costs more.
I've been eyeing 6.5 CM for a few years but I wanted to make sure it was going to see widespread use and not become another orphaned proprietary cartridge. As far as I can tell 6.5 CM is pretty mainstream as of 2019 with all the major US ammo companies making ammo for it and most or all of the bolt-action rifle makers offering multiple models in it.
I really don't need this change, but I kinda want to do it, and am simply looking for any issues or anything I might have missed. Remember that I'm not reloading so some other good options like .260 Rem, 6.5x47 Lapua and 7mm-08 aren't as practical as 6.5 CM, 6 CM, and .308. And I'm not looking as seriously at 6 CM because of the barrel life issue and more limited ammo options. Understanding that barrel life varies, what I've read here and elsewhere suggests my .308s might have an accurate life of 4000-5000rds, a 6.5 CM might be around 3000-3500, and a 6.0 CM might be closer to 1800.
Also, while I have no options beyond 600 yards anywhere local, in 1-2 years I'm hoping to take a 1000 yard course at some place like the Michigan Training Center or NRA Whittington Center. In .308 the Superformance ELD would probably do OK at 1000, but there are several 6.5 CM options that look like they would do better.
The whole point of this is to see if I'm making a mistake before I spend the money, so tell me if I am.