I use a Visor Deluxe (of course see sig), which is very similar to the Palm IIIxe but with a slightly older OS, but also with an expansion port. My fave is the Visor Platinum. I'm not a big fan of either the Palm IIIc or Visor Prism, but the Prism wins out there. The PocketPC machines are better suited for colour, but I don't like the high price.
The TRGpro is fine, but it doesn't have as much support (but it is Palm OS), and it costs quite a bit for what you get.