I don't like convincing ppl to buy a certain Cal. or name or model. I prefer to give as much info as I can and allow you to make your own informed decision. That's the way my FFL has always been with me and it's prolly the reason I use him exclusively.
The .300 Win mag is faster, more powerful and can be more accurate than the .308win.
But what does that mean in real life application?
Shooting the exact same bullets, the .300 is on average 300fps faster, has 300-400 more ftlbs power but also has 1.5x as much recoil. At 500yds thats about 15" of difference.
In a hunting scenerio there isn't much advantage until you get into 500lb+ animals. Even then shot placement trumps power.
In a Tactical/match scenerio the 300 steps up at longer ranges, the heavier 190gn-240gn VLD, AMAX and MatchKing bullets are to long to be loaded in the .308 case. But these are extreme bullets made for extreme range; that kick like a bitch and must be handloaded. If you don't load they don't help you.
The .308 is a short action which is quicker to use and shorter is stiffer meaning more accurate with Equal bullets.
Less recoil in the 308 so less flinch and less pain so you shot it better and more accurately.
Ammo, non surplus is around 5$ a box cheaper for the .308. more trigger time
Now your asking thig question so i'm guessing you don't own a .30 cal rifle. To take advantage of either of these guns long range ability you need to spend a few buck on glass and not the shit you find in cheaperthandirt or the sportsmans guide etc... but a scope thats costs as much as the gun. No offence here but do you think your good enough with a 30 cal to take advantage of the .300's long range ability 600+yds?
Theres so stuff for you to ponder,