I think Palm-based PDA's are on their last leg. I was a devoted user of palmOne handhelds (I've had the Vx, m500, m505, Tungsten T2, and now the Tungsten T5).
I switched briefly (a year or so) to an hp iPAQ 5555, which I later switched over to the 4155. I loved the 4155. It was very small and very reliable. Pocket PC's seem to be more stable and offer more features now.
When I switched from Windoze computers to Macintosh computers, I had to change back to a Palm-based PDA. I bought the Tungsten T5, which has a lot of neat features that weren't on the Pocket PC. Initially I loved it, but I have to admit, it's not a quality palmOne product. Pros: very nice display, large memory, super fast, lightweight. Cons: big, buggy, and doesn't include PalmOS 6 (what the crap is up with that palmOne?).
The fact that palmOne hasn't included version 6 of PalmOS makes me wonder if palmOne will be switching to Pocket PC software for their future generations of handhelds. PalmOS 6 is powerful, moreso than PocketPC, but none of the PDA manufacturers seem to be rushing to install it on their handhelds. Chances are this will not be available as an upgrade since PalmOS 6 has so many changes (it's basically an entire rewrite of the operating system).
If you have a Mac, then you can get PocketPC's to work by using MissingSync, but most stay with Palm-based PDA's. I like Palm's, and I like PocketPC's. Each has their own unique differences. Go to a local store and check them out. If you decide to go with a Pocket PC, then Dell is probably your best bet. I wouldn't recommend ordering one until you've at least tested a few Pocket PC's at Best Buy, CompUSA, etc.
I've heard rumors that Apple may be re-entering the PDA market. We'll have to see what their products are like, but if the iPod is any indication, then it'll probably make its presence well known in a short amount of time.