I hear and understand the two most common reasons why someone should pick the big Winnie over the WSM, but I aint buyin it.
1). Because more ammo is available for the 300WinMag: Well sure it is, it only has decades on the WSMs. BUT Federal and Winchester are steadily adding WSM ammo. I think its only a matter of time before you see Hornady and Remington throw their cards into the pile (Rem has already dropped their RSAUMs in the rifles and the ammo selection for them is almost nil, Hornady likewise is one of the fastest growing commerical loaders. Why would they not want to get aboard THE fastest growing calibers in the business?) Also how many hunts do you go on and forget your ammo? While it might be comforting in the back of you mind, I for one would NOT be concerned with not being able to find ammo while on a hunt away from home. Are people really that forgetful? I have heard of forgetting somethings while packing for hunts, but I know I have NEVER forgotten to pack my gun, boots, ammo, knives and optics!!!
2). Because you can stoke up the handloads. You can say that for just about ANY caliber out there. Likewise if you are a handloader than the 1). excuse likewise takes you out of the loop regardless of whether you have a 300Winnie or a 275 H&H.
The 300Winnie is a great ol warhorse, and if you have a reason to shoot 200 or heavier grain bullets (like at grizzlies, which then I am gonna carry more firepower!) then stick with it. But simply a 180 grain quality bullet from either the Winnie or the WSM will plant ANY moose or elk on the continent. The advantage goes too the WSM for being in a shorter, lighter package that is a short action. I REALLY like a light, quick handling gun when packing up in the rough stuff. The short action likewise makes for quicker follow up as you dont have to take your head off the stock while cycling the bolt if you need a quick second shot (hopefully you dont, but you want to always be prepared!).
Good luck with either, but just dont rule out the WSM just because of tradition. Who knows, the WSMs have already exploded onto the scene at an unheard of rate. 30 years from now they might be as standard in the deerwoods as a 30-30 was 50 years ago and the 270/30-06 is today!

RA