I own both the SC-55 MK1 (with the version 2.00 ROM, if I recall correctly) which is the one with also the GM logo.
I've owned an AWE64 since December 1996, but was never impressed by some of its MIDI samples (particularly the trumpets).
Since most games of the early 90's were written on an SC-55, I decided to "bite the bullet" and bought my SC-55 a couple of years ago (I think I paid back then about £65 for it).
My SC-55 is still in very good condition and has been well look after by its previous owner.
I'm very satisfied with the MIDI sounds of this unit (from a gaming perspective), except for maybe one or two instruments.
I little while later, by a stroke of luck, I came across an SC-88 Pro and managed to buy it (locally) for about $35.
However, this unit has seen some "abuse", but the unit itself is still functioning fine.
I compared the sound playback of the SC-88 Pro to that of the SC-55 and, for the most part, they sound very much the same (in terms of quality of instruments).
However, there are differences in some instruments that I could pick up.
For example, when I played back the Doom E1M1 soundtrack, I felt that the electric guitar sounded much better on the SC-88 Pro than on the SC-55, but this is based on my own personal preference.
I bought the SC-88 Pro purely as a "backup" unit in case my SC-55 decided to stop working one day. So, at the moment, it's not in use.
But, I might in the near future start it up again and play around a bit more with it.