Only owned 1 300 WM and liked it. Only 7mm is my current 7-08 that I'm waiting on components for.
That said - I suspect that the difference between the two is similar to what I found about the step from 30-06 to 300wm.
If I need to move up, 300 wm isn't really enough. If the -06 REALLY isn't enough, I'm probably in 338/375 territory.
I can't imagine a place where the 7mm isn't enough but the 300 is - terminal wise. There might be reasons why you'd want one over the other (162 AMax being one of them) but I doubt it's critical.