I would assume that the 300 WSM is going to be around for a long time. And the 300 WM has an excellent reputation/history as a hunting cartridge. With that being said, I would get the 300 WSM. But, if I had a 300 WM, I won't go running out an swapping it for a WSM.
I honestly would pick the one that feels best to you. That way you will have more confidence with it.