The .300 WM seems to have a slight edge over the .300 WSM, on paper. In practice, I don't think the difference would be noticable, except the WSM would have slightly less recoil. If you reload, you would use less powder for the WSM, giving a slight savings over time. The WM is belted, and as such, headspaces on the belt, as opposed to the WSM which headspaces on the shoulder, making it inherently more accurate, but again in practice, I doubt the difference would be noticable to the average shooter.

If I were to go out and buy one of the two today, I would choose the WM, but only because I am more of a traditionalist. But that's what it all boils down to, is personal choice. One really isn't all that much better than the other.