The two have almost identical ballistics, recoil energy, muzzle v., etc.
I think all would agree that both are more than capable of bringing down all North American big game (even griz).
So what are the intangibles? Assuming the cost is equal, you say the WSM "fits" better. I'd still take the WM and cut the stock down, save it, and glue it back on when he gets bigger knowing that I will still have a better resale value in the WM than the WSM. The only thing I can see that the WSM or any short mag gives you is a shorter action which should allow faster reload and shooting. Who cares?!

I still say gun makers "invent" rounds just to improve sales, not improve hunting success. Which is ultimately why we choose a particular gun. All magnums have dramatically changed the way we look at rounds. They provided "flatter" shooting projectiles and heavier forse at contact. The .300 WM is tried and true. I'd stay with it.
Some can afford to match a weapon to the game and own several guns, me . . . I got my .300 WM and take everything with it. Antelope, deer, elk, bear, moose. Even thought about taking it goose hunting, but it's against the rules!