I'm not 100% certain, but I think the MKII also has more EQ control. I had an original 488, but it only had a treble and bass tone nob for each track.
The 488 is a great machine though. The only downside for me was that you can't record more than 4 tracks at a time on it, and I was mostly recording my band. For a singe person - aka a one man band setup - it's a great machine.
Actually if you are looking for a feature filled 8 track portastudio don't forget the TASCAM 688, which is the best and most feature filled cassette based portastudio ever made. It is truthfully a bit more complex and difficult to master than the 488 & 488 mkII however. Of course if you already have an appropriate mixing board the TASCAM 238 is also a great machine that is very easy to use.