Don't get a WSM just for the sake of doing it. Start with requirements and drill down to caliber from there:
- Reload?
- Target or critter getter
- Have parts already on hand?
- Max weight of rifle
- Desired effective range
- Remaining energy at desired range
As far as magnums go, it's hard to beat either the 7mm RM or the 300 WM, but you need a long action with a magnum bolt face for them. For short action magnums, 7 WSM has phenomenal ballistic performance with the heavies, but you will probably need to single feed 180 VLDs in a short action. 300 WSM is a fine choice if you need a very light mountain rifle that delivers something akin to a strike from Thor's hammer on impact. The 300 WSM is also a better choice if you don't handload.
The cool kids on this forum are all liquored up on the 6.5 SAUM. With 130 VLDs, it is probably capable of taking down most anything inside 500 yds and can be one heck of a target rifle too (only 6 mils elevation at 1000 yds); plenty of annoying case prep involved.