The CD ones only have a map per region, UK & Ireland, France, Spain, Italy. The DVD ones have a map for all of Europe for example. The CD maps are cheaper then the DVD ones unless you want all of europe.
One difference I am aware of regarding the DVD units is that some cannot play CD's with out the multi-CD changer installed, you need a firmware upgrade to enable audio CD's, though I think its just with the early units. The CD version and I believe the DVD unit cannot play MP3's from a burned CD. If you really want the DVD version you could get an Ipod adapter and have your music that way.
Finally the DVD versions screen image is slightly different ie the buttons disappear when not in use.
Hope this helps, my CD version with ipod adapter is fine and was a bit cheaper then the DVD.
Dave N