Danny...

i would personally go with the 300WSM if you are going for a short mag. seems that the 300WSm is gaining so much popularity that you are seeing alot more bullets from federal and other companies than the 7mm WSM.
it might be also easier to find ammo for the 300WSM than the 7mmWSM. i have not bougth one yet ( waiting for the finnlight in 300WSm ) but also because i wan't to see if the ammo selection becomes more broader