big water pan, small charcoal basket
locking/removable/stackable sections with handles for portability and ease of access to meat
only one bottom vent
smaller diameter than the WSM
cheaper construction than the WSM
smallish water pan large charcoal basket
only one middle section without handles
three bottom vents
built like a tank
I have used both and honestly, I gave away my Pro Q. In terms of ease of use, the WSM is pretty much set and forget. The three bottom vents give you excellent control of your burn, the Pro Q, not so much. This was my major concern, if you have control of your fire, everything else is gravy. I couldn't seem to keep temps up, with the top and bottom vents wide open.
They both put out excellent BBQ, I just ended up sticking with what I was accustomed to. Since giving away my Pro Q, I have picked up another WSM.