Although I like the WSM cartridges (I own 2 - one in .270WSM, the other in .300WSM), I must admit that they are essentially really no big deal. However, the new WSSMs, especially, the .223 WSSM, are even more about hype than practical advancement.
I say this because, while the .270 WSM has noticably more velocity than the regular .270, the .223 WSSM is not that much faster than a .22-250 or .220 Swift.
Then take a look at the other pros and cons. With the regular .22-250 and 220 Swift, you have a plethora of available factory ammo choices. With the WSSM, however, you are very limited.
The same holds true for rifles. While there are countless numbers of makes and models that chamber the .22-250 and .220 swift, there are only a handfull of makes and models that chamber the .223 WSSM.
Unless you want the .223 WSSM just because you really, really want it, your purposes would be better served with a "tried and true" .22-250.
Zachary