A used RIO is better than a new Dension. I own a RIO and am looking into the Dension because Dension offers a CD-changer interface so I can install into my Lexus (which doesn't have an AUX input like my Pioneer headunit installed in my Honda Accord.)
IMHO, find a used RIO and get it and hold on to it, upgrade the software to the beta (stable and much better), and upgrade to more capacity if it's only 10GB. The MP3 tag search, visuals, and ethernet connectivity make it a stable unit with fast downloads. The thing is made very well, and it never skips with the laptop drive (not like a 3.5"). It doesn't overheat like the Dension unit has a problem doing.
The Dension does offer only a few advantages, but not enough to outweigh the Ferrari of MP3 players: RIO Carplayer!