From the looks of it, you definitley have to go with a Microsoft based PDA. That leaves you with Asus, Dell, and HP. I have no experience with the Asus PDAs, but the Dell Axims and the HP iPaqs I do. IMO, the Axims are far better; better support, less expensive. Based on that I'd reccommend you go with an Axim and check out aximsite.com if you need suggestions on which specific model to get.
I have an Axim X30 high. They just came out with the X50, but don't get one of those unless you need either a CF slot (SD cards are just as cheap nowadays, but CF have more functional cards other than just storage), or a VGA screen (which only the high X50 comes with). IMO, it isn't worth it. So that is what I reccommend, and X30 medium or high.
If you want one for use with T-mobile service, check out one of the iPaqs. Get all you gadgets bundled in to one that way.