The 300WM, and the reason being: it shoots smaller bullets. Wierd, I know, but let me explain.
The .338WM is a great cartridge, and it does give you the ability to use bigger bullets, but the 300WM is going to let you use smaller bullets which is better, for a couple of reasons.
One reason is that it is in a smaller package, making it more packable.
The second reason is that with the .300, you can get smaller loads for use on smaller game. You never know when you are going to take a 200lb black bear, and you aren't going to want more than a .300WM, or you are going to start destroying more than killing them.
I do know a lot of people who do hunting with the .338WM and love it, but you don't need more than the .300WM. I mean really, this is something that comes down to preference, but what if on your southeast bear hunting, you are running into 400lb black bears, and then take some 130lb blacktail deer? That .338 is gonna do more damage than the .300 on those blacktails.
In all reality, unless you are hunting bison... the .30-06 is all you really need, but you can't go wrong with a good .308 caliber.