Well if I had a 300 Win Mag (which I did), I would not run out and buy a 300 WSM. However, that being said, I don't think there is anything the Win Mag can do that the WSM can't. And If I was going to pick one, I would go with the WSM. I don't believe the ammo is an issue. The WSM has gained enough popularity in its short history that ammo is readily available. The rifles for the WSM are of equal quality as the Win Mag but are usually lighter. Personally, I think the WSM may have a slight edge in accuracy because of the more efficeint case. But the old Win Mag can be jusiced up a bit more throught reloading.
But its 6 of one, 1/2 doz of the other. Take your pick and pick the one that fits YOU better.