Ok... you asked, here is my advice.
First off, I'd say go with the Palm OS. However, I'd skip past the Palm Z22 because the screen resolution is so poor (I had one).
I've got a few friends that, over time, migrated to the Windows based PDAs and have come to regret doing so. I'm NOT saying Windows based PDAs are "bad". I am saying that in my experience, over time, most have regretted going to Windows. My Palm T/X boots in half the time of most Windows based machines and has never locked up or "lost software".
I'd stay away from PDAs with bundled software. Why? Because you will pay a premium for what someone elese thinks you need. Only you know what you need in your particular circumstance.
My recommendation... Palm T/X (yep, its the most expensive of the Palms, but its has a beautiful 320 x 480 screen, handles wi-fi very well so email is fast and easy, never crashes, is expandable via SD card, can switch to portrait mode for showing images, plays MP3s... it is a "do-everything-well" machine.
Next is the software you will need. I have found medical/nursing info www.medicalwizards.com
to be GREAT! I can't say enough good about their products. I use their ER/ICU Suite v 6 everyday at work and I really appreciate it. Their programs are very reasonably priced as well.
There are some things you just shouldn't scrimp on... a fully functional PDA can be your best friend... Palm has always made my life easier and combined with medical wizard software, I can calculate a weird drug concentration for an unusual body weight patient 2-3 times as fast as anyone else... and listen to favorite MP3s in the break room on all those breaks RNs get ;o)