If you're looking for personal experiences with the two phones, here's my $.02 worth...
I was looking at the same two phones. Having been a previous HP iPaq user, I was really leaning towards Treo 700wx to get the full PDA experience in my phone so I wouldn't have to carry around 2 devices anymore. The Treo had a familiar interface with the touch screen and all, but to be honest I don't miss it in the least and can actually say I prefer not having it in a phone. I took the time to think about any features that the Treo would give me over the Q9 and then decide if I would really take advantage of them on a daily or even weekly basis. The final answer was no. The one thing that had me stuck for a bit was that the Treo offers "full blown" MS Office application versions, but unless you really need it because you do business it's not that big of a deal since DocsToGo gives you enough.
The Q9 is lighter (only 1.2 ounces lighter but it makes a difference), has a bigger screen, (320x240 compared to 240x240), has more talk time (7 hours vs 5 hours), I think the keyboard is much better (wasn't impressed with my dad's Treo, too cramped), and the Q9 is just sleeker (maybe a bit wider, but overall not as clunky especially with that antenna on the Treo).
Ultimately it comes down to personal preference, but those are the things that I observed going through the same decision process as you.
I chose the Q9 and haven't looked back!

(oh, and I threw the iPaq in a drawer and my Q9 is now my PDA too ;) )
Hope it helps and good luck!