Looking at the numbers, it is virtually identical to the .25-06 - which is THE .25 caliber cartridge. (Granted, the .257 Wby is more powerful, but the .25-06 is the most popular of the .25s.)
That said, I see no real advantage to the .25 WSSM. Now, if it was a .25 WSM, hence with more velocity - close to a Wby, THEN I would find it more interesting.
It's kinda funny how I like the .300WSM and kinda justify it over the .300 Win Mag, but don't do the same with the .25WSSM. Thing is, there are very few rifles being built in the .25WSSM and same goes for ammo, where as it's not the case with the .300WSM.
I don't necessarily want to sway you away from the WSSM, but I do want to let you know that you can't go wrong with the 25-06.
Zachary