Since nobody else is weighing in: I'd characterize this is an ergonomic issue as much as anything else; the main question is which form factor you like better, both the Treo Pro and most of the HTCs should be able to run Pleco fine. The big tradeoff with the Treo is the smaller screen, which tends to make Pleco's user interface feel a bit cramped (though this can be helped somewhat by using the experimental full-screen display feature). With the HTCs, you can either give up the hardware keyboard (though you can still tap out letters with a stylus using the onscreen one), or have that keyboard but at the expense of a much bigger and not-very-pocketable form factor.
If you go with the HTCs, you might want to hold off on purchasing for a few months, since I believe the new HTC Touch Pro2/Diamond2 are scheduled to hit Verizon later in the year. Oh, and make sure the phone you get has a touchscreen - a few HTCs (including the "Ozone" on Verizon) don't have touchscreens and hence aren't capable of running Pleco, you want one with "Touch" in its name. The Verizon "XV6900" is actually the original HTC Touch and works fine with Pleco, so if they still have any of those in stock that might be a cheaper option for you.
One other Verizon phone you could look at is the Samsung Omnia - opinions vary on that, but a lot of Pleco users seem to be very happy with them.