The 'Hi' in Hi-8 stands for High Band. The main difference is that Hi-8 cameras and players use 'metal evaporated' tape which allow more lines of image resolution to be recorded.
A Hi-8 recording has about 400 lines resolution of recording, whereas 8mm only has 260 Lines. This means 8mm has a visibily inferior picture quality.
However, both formats are now largely obsolete, and are definitely not worth bothering with. They have been superseded by mini-DV.
Imho i find the hi8 and 8mm are pretty good for family use... I feel safer with a tape based machine than with a disc based machine at this moment in time.... I bought a 8mm 15 years ago and can still pop them into my hi8machine and can still transfer them to vhs....and they will be viewable as long as people keep with vhs and as that format meets its doom I will have them transferred to digital... Sony has just announced that it will release a minidv camcorder next autumn based on its blueray technology.... so todays format will be out of date with 5 years ....they will probably be able to hold a Terrabyte... and a scratch on dv discs can make them unreadable...a mangled tape can be cut out and glued together....