Why choose a 308 over a 300 WM.
They both shoot the same exact bullets, just to different speeds a 300 won't quite be 500 fps faster than a 308, 300-400 fps is about right depending on the bullet weight, but the difference in performance is less than many realise.
Lets use elk hunting as an example. It is generally accepted that if a bullet retains 1500 ft. lbs. of energy at impact that is considered the minimum for a clean kill. If we accept that as a fact, then with the best long range loads I can find a 300 WM still has that energy at 675 yards, a 30-06 at 575 yards and a 308 at 475 yards. I used a 200 gr Nosler Accubond with a BC of .588 to calculate this from a 300 mag @2850 fps, 30-06 @2650 fps and a 308 @ 2500 fps.
At a glance the 300 has a decided edge, but realistically, few people have the skills to take advantage of the extra range. For most people 300-400 yards is about as far as they have the skills to shoot. Many far less than that. At those ranges no elk will ever notice the difference.
The 308 has other advantages. It has roughly 1/2 the recoil, around 16-17 ft lbs vs 28-30 ft lbs in the magnum. Rifles can be much smaller and trimmer, mag capacity is 2-3 more rounds, and most people find they can shoot a 308 more accurately. Having the best rifle is not always about having the most power.