Deer rifle..

[CS1983]

6.5CM, for sure. Overall better performance and more capable for LR.

I wouldn't count on .308 being as obtainable, and I'd certainly not wait til September to look for it. You should make the caliber decision and be buying anything you can find regardless of the decision you make.

6.5PRC is a very LR capable round, but I would think it a bit ridiculous for deer and pronghorn at the ranges you are talking about.

[Little Dutch]

6.5 CM wasn't yet introduced when I bought my hunting rifle. I likely would be hunting with a 6.5 instead of a .308 if it were. Nothing wrong with the .308, but the 6.5 is a good cartridge. Neither one will be in stock or readily available before hunting season 2021. You could flip a coin and be fine either way though, assuming you can find ammo or loading supplies.