The step up to a VP150 isn't much, I have both and other than a very, very slightly lower tone, a very slightly(1db) better off axis measurement and the ability to play louder there is no difference between the 100 and 150. Get the best sub you can, much more bang for the buck.

Why do I get the feeling a VP180 (Dual 6 1/2" drivers flanking a vertically oriented tweeter and 5 1/4 mid) is in the works?

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70