The accuracy over velocity argument isn't a valid one; the .300 win mag does shoot faster than the .308, thus the higher kinetic energy, but they are equally accurate cartridges given the shooter.
You can't shoot both .308 and .300 win mag cartridges in the same rifle. Don't even try. Would recommend you look into SAAMI specs or even just a google image search of the two cartridges side by side.
The only reason the .308 is more popular generally speaking is because of ammo cost. .308 / 7.62x51NATO has been around a long time; one of the most well documented cartridges available.
NRA fo Life Member