For 600 yards and less for deer size critters I will grab a 308 over ANY other cartridge any day.
Dont be fooled by "paper ballistics"
I know that magnums offer less drop and less drift ect....but with the cost of operation, barrel life, recoil ect.....You can practice with the 308 more often and without learning to flinch. When you practice regularly with a given weapon you become familiar with it enough to make clean kills regardless of drop and wind. Dont get caught up bigger is better. With its accuracy and my knowledge of it, I was able to drop a bull moose in his tracks at 438 yards with the 308 using ONE shot. My buddy had shot at him at 390 yards near dark and missed him. He was just getting ready to nose into the brush and stopped for a quick look. This is only one story of many that I give credit to my 308.
I do not "know" any other of my rifles like I do my 308. That is because with the short bbl life and HUGE amounts of powder burned along with the need for well constructed bullets which are expensive, I cannot afford to shoot my 300's the same amount as my 308. You may ask, then why do I own 300 mags? Because I need the retained velocity to reliably open bullets beyond 800 yards on game. My 308 will not do that. However for 600 yards and less, it does just fine. The all around accuracy and practicality of the 308 more than makes up for a minor energy loss. You will enjoy owning and shooting a 308 more than a magnum. Unless youre making up for something!
Now if you want to shoot bigger critters or deer at LONGER ranges than you specify, then it is time to concider a mag. There is always a price to be paid when doing something extreme. An elk at 800 yards is extreme. You will have to pay the price. That price is more recoil, less bbl life, more expensive powder ect....But if you stick to the critters AND the ranges you specified, the 308 is more than you will need.