AFAIK, there's no difference at all. You're talking about the current versions, I assume.
In fact... interesting bit of info...
The Power Wah's PCB has a lot of empty space on one side. It also has a gap where the LED for the volume section would be installed (if the board were used for a PWOV), and there's a printed checklist with checkboxes for multiple models.
Basically... all they do is use the same board and fill in the spots for the volume section. So, there's no difference other than the extra feature.
I'm actually not sure how the volume section is set up... so I really can't tell you much about that. But I know plenty about the wah section.
Oh, and if you're talking about the old ones... the same general realities apply. Same thing, no difference in wah.