I know I'm pulling this number out of my ass, but I would say 99% of games with General MIDI support will sound the same on these modules.
The MKII does have extra Instruments (but outside of the General MIDI standard) and can handle 4 more voices, but does any game take advantage of that?
Unlike the differences between the MT-32 (Old) and let's say a CM-32L, I don't think it matters much what SC-55 you pickup. I do have both, just in case, so always happy to help out with recordings if someone needs one.
I think I did some for World of Xeen, but can't remember if that was on a SC or on a CM-32L.
EDIT: Here is that thread:
Xeen and Roland synths