"Wider varitey of factory ammunition" ???

This used to be the case, but not anymore. Federal has 8 factory offerings in both WM and WSM. Winchester had 7 WM and 8 WSM. Availability may be an issue now but in the short future will change. My local Cabelas was sold out of WSM and Sportsmans warehouse had very limited supply. Both had ample WM. Online Cabelas was sold out of WSM and had pleanty of WM. Not sure if its that they stock more WM or the WSM is more popular, I think the later, but thats my gut feel. I am a huge 300WM and 338WM fan, but went with the WSM because of a lighter rifle, less bolt movement, new technology, same balistics from factory loads.