Few important things I should think of for choosing your dictionary:
1)Pen input
Writing a Chinese character in to find how to read it is easier than doing it other ways: PDAs or smart phones normally has a better interface.
2)Your level of Japanese? ( How soon will you upgrade the device?)
At some point, you should switch to Japanese to Japanese dictionary because you will spend more time reading Japanese that way: bilingual dictionary may help you with understanding grammar etc but you will spend less time thinking and reading in Japanese.
3)Price: just in case you lose or break it
Dictionaries can break or be lost. Particularly, if you take it to school every day. it is not all that unlikely that you lose it or break it. Maybe you would like a relatively affordable product.
4)Using in Class room (How to avoid looking like playing with a phone)
Smart phones PDAs are great but I don't know if you want to use them in class room. Maybe something that look like a dictionary is better sometimes?
5)How will you use it?
Not like electronic dictionary would help you a lot in class, because you simply won't have the time to look up things. If you can't understand teachers at all, however, you may end up spending a good amount of time reading text books with your dictionary.
My advice
Get a smart phone or PDA to be used in private
Get a cheap Electronic Dictionary or paper dictionary to be used in class room
Our electronic gadgets will evolve a lot this year, so if you are not coming to Japan this spring, then maybe you should pick a tablet PC (
Entourage Edge e-reader shows off its softer-ware side on video | Engadget) or Ereader like sony PRS-600(Currently no Japanese dictionary with pen input available yet)
Japanese Language Tools
http://www.bornplaydie.com/japan/dictionary/reviews.htm