Is there an appreciable difference in sound quality between the SC-88 and SC-88 Pro?
Well, I had a SC-88 Pro ST and a SC-880 for a short time and I didn't test them very much, but yes, there is a noticeable difference in sound quality. But not only that, but the effects section is much more poweful in the SC-88 Pro/8820/8850 (more effects and more effect units). And I recall having read somewhere that while the SC-88 is the combination of JV-30 (SC-55) and JV-80 waveforms, the "new" 88 Pro waveforms come from the JV-1080 (Roland's flagship synth module at that time).
The downside is that second hand SC-88 Pros are VERY expensive (usually no less than $250), and for the price one could consider getting a JV-1010, a XV-2020 or something simmilar (unless GS compatibility is mandatory).