I'm pretty sure the 300 wssm is a wildcat and brass is practically impossible to get.
300 WSM on the other hand is going to be around a long time and it is offered in most brands of rifles. I'd go with a 300WSM if I were going with a 30 cal. It uses less powder than the 300 WM to achieve similar velocities so it is a more efficient cartridge. Also, being a short action, it is usually going to weight less than the 30-06 or 300 WM rifles.