The main difference between Mini DV and Digital 8 is the tape size. Digital 8 was intended to be an easy way for 8mm/Hi8 users to move to digital. The specs of the format it's self are very similar to Mini DV. The cameras are only consumer grade entry level cameras but produce good images if used properly.
In my experience, the MiniDV format surpasses the Digital 8 format to a certain degree. On a standard, midrange Digital 8 the picture quality is visualy exactly the same as that of a standard, midrange MiniDV, however MiniDV has one feature that I have as of yet to see on a Digital 8 camcorder-Progressive Scan frames. This feature gives your videos a much more professional look when watched on a television monitor, as it appears quite similar to motion picture film. This is the only real difference I can see between the two different formats, so if you can find a camera with Progressve Scan mode, get it.....it's worth the extra bucks.
Here is a link to help some more.
http://hometheater.about.com/library.../aa020300b.htm