For software, I use Lexi-Complete, available for $150 through pharmacist.com. It runs on all PDAs.
The PDA itself doesn't make any difference. Just look for the features that you want. I got the one with the built-in Blackberry style keyboard. It's easier for me to type, than to poke with a stylus. Mine also has built-in Wi-Fi, which I don't use. The newer model has a built in MP3 player, which I also would not use. On rotations, I just need access to drug info.