I shoot both 6.5 CM and 308 Win, but I only have 2, 6.5 CMs and 8 308 Win. It's all about what you want to compare...the 308 can easily beat the 6.5 CM with handloads compared to factory. 6.5s. I ran 17,000 matchoads in 308 out to 1400yds, with standard loads of the day, and it would hold half MOA. Today new powders and better bullets have changed things. The 6.5 CM works well with hybrid cases and RL 26 to run 150 SMK at 3000 fps. But most of the rest can be beat with a 308 Win, with some of my loads reaching over 3600 ft/ lb of energy in Lapua Brass, switch to hybrid csses and go to 3800 ft/ lbs.
Depends on how you set the 308 up, I've gone to 8 and 9 twist, for .715 to .823 BC ...unavailable in 6.5. So without my hybrid cases for the 6.5 CM the 308 has more performance, available, and a bunch of more energy.
I use neither for hunting...I've only used large magnums for all game, for about 50 yrs. Unless it's varmints then the 308Win and 222.
Here is a 6.lb 3oz 16" 308 AR 10 with more energy and much better accuracy than my heavy 24" AR 10 in 6.5CM weighing 12 lb. The
little 308 win hit 2900 fps with some 155s and 2756 fps from the 168 eldm from a 16" barrel a 6.5 CM 16" will not do the same.
So it all depends on if you are a fairly advanced handloader, do your own gunsmithing, alot of things are possible to really increase the output of the 308 case. But for the average guy or non handloader the 6.5 CM is a good option...better depends on what you're doing. I personally have no problem choosing the 308 Win over the 6.5 CM. Back in the day I went 8000rds before changing out barrels, the 308 has cheap components, and ammo can be found everywhere, even on the ground. If I want lighter than the 308 for targets I go for the 6 mm. But I also have a new barrel for one of the 6.5 CMs haven't given up on it yet, but its low on the list of of my cartridges. Got rid of the 7 mm, cutting down on some building others...it's what we do. Whatever works for you...