Well first of all 620BT has integrated Bluetooth (short range wireless connectivity, to use with cell phone etc.) and I think it has 32MB of extra ROM space to store application etc.
For the $50 extra I'd definitly go with the 620BT.
Have a look at this thread, some info there:
http://www.pocketpcthoughts.com/foru...ic.php?t=20053
Good luck!