I have an iPad2 and an iPhone4. I'm in love with them both for so many different reasons!! There are tons of helpful apps for both devices like drug guides, medical dictionaries, nclex practice questions, etc. Also, my favorite thing is the Tegrity app. All of our nursing lectures are recorded by our instructors and are uploaded unto Tegrity. The app makes it so simple to carry my lectures around everywhere.
iPod Touch or iPhone pros: wi-fi, easy web access anywhere there is a wi-fi connection. (iPhone has 3g so you can have internet anywhere), helpful apps, very portable
iPad pros: larger screen, portable, easy to take notes on if you are used to the touch screen keyboard, basically the same as the iPod/iPhone only larger, also has wireless and 3g capabilities (depending on which model you get), can download notes and print using a wireless printer directly from the iPad, can also use your text books online if your textbook companies offer it, oh and it has an extremely awesome battery life.
I have honestly only used my lap top twice since we got our iPad 3 months ago. That was when I needed to upload some picture from a CD. (The iPad doesn't have a disk drive, although there are rumors that they are coming out with an attachment for one, not sure how true it is though)
Hope this helps some!!
My husband has also looked into getting a Samsung Galaxy tablet too. I think they are supposed to be pretty useful too, but I'm a total Apple geek. I was already familiar with the userface. There are also more apps offered for the iPad, and less chance of iPad/iPhone getting hacked through apps. (all apps from the App Store have to be aproved by Apple, so there's no chance of 3rd party apps that run viruses.)
edit: I should add that while in clinicals I use my iPhone or iPod touch and in class I use the iPad.