7 mag would probably be the best choice, you can find ammo anywhere. I would say if you really want a 300WSM and the thought of not having ammo that you can just pick up anywhere doesn't bother you, go for it.

Ammo concerns don't really factor in too much in my choices, Midway USA carries everything and can get you what you need in short order, if your local gun shop can't get it by the time you need it.

A little pre-planning and you'll always have ammo no matter what you shoot.