The main benefit of the VP150 seems to be in larger rooms. It can play a bit louder and (presumablly because the drivers aren't working as hard) is reported to sound more "open" at high volumes.
People who A/B'ed VP100 and VP150 in small/medium rooms tended not to hear any difference. People who A/B'ed in large rooms tended to think the VP150 was better in the ways described above. If you have a "wide angle" listening area (wide but close to the screen) the VP150 is also supposed to be a bit better because of wider dispersion.
I have a VP100 myself and am very happy with it.
If you're looking for something "better" (albeit more expensive) the high end Paradigms (CC-470 and up) and the Rocket Bigfoot both get really good reviews but do both cost quite a bit more than the Axioms.
The Bigfoot is supposed to be the better of the two but since the Bigfoot is essentially an M60 layed out in center channel configuration (a) it's kinda big and (b) it's hardly surprising
I doubt a Bigfoot will fit in your cabinet but the Paradigm might. I don't think you will find a big "sound quality" difference between the Paradigm and the VP-100 but IIRC the woofers in the CC-470 are larger (more like the 6-1/2" woofers from Axiom) so the CC-470 seems to be able to play better at high volumes than VP100 as a result (as can a VP150).