Check out the technology forum. There's a few links about pdas.
http://forums.studentdoctor.net/forumdisplay.php?f=109
I don't believe there's a lot of med software (ie. epocrates) out there for blackberry at this point. You're probably going to want to look at Palm or Windows Mobile devices to have the largest selection of med software to choose from.
Lots of opinions out there, but in a nut shell, Palm is a stable, solid OS, but hasn't been updated in quite some time. Windows Mobile maybe a bit more buggy, but is frequently updated, more eye candy etc. The other thing to think about is whether you want a smartphone or pda only.