The University of Minnesota gives each new resident the Palm Zire 71 (the blue one with the camera).
As far as buying your own, it really depends on how much you care about extra features (or if it's for work only). Most medical reference/calculator programs are really basic, and don't require a fast processor or color screen. Some programs, like the 5mcc series on Skyscape, incorporate color pictures, though, and you will get no benefit from a monochrome screen. Some residents like to have a PDA with a camera so that they can take pictures of exam findings at the bedside and share them during case presentations. If you want to do this (and the institution allows it), you will want a better, faster model (Palm Zire 71 or Sony Clie models).
I own a PalmOS PDA myself, and have no experience with PocketPC PDAs, so I can't comment on those. I hope this helps.👍