RE: what is the "real" difference or benefit?
I personally wouldn't put WSM and WSSM in the same catagory. I believe the WSMs (at least 270 and 300) are here to stay, but I am not so sure about the WSSMs. My rule of thumb for using a caliber is that I have to be able to buy ammo at WalMart (it may not be the exact grain or brand, but it would get me by in a lurch). That being said, I have never failed to find 300 WSM or 270 WSM at any WalMart that I have been to. I do own 2 300 WSMs, but you can do the same job with a standard 300 WM and have a larger choice of bullets and weights.