Keep in mind that in game usage both 88 and 88pro are pretty much limited to the SC-55 map... there could be some tracks that benefit from higher polyphony but other than that don't expect anything else.
I have a 8820 and I have tested it in games with all 3 maps: 55/88/88pro , some instruments could differ between them depending on the track playing, some sound "better" with 88(pro) modes, some with the 55 one. (still, better is in quotes, a better word could be: different)
Now, the 88pro has an extra fx processor AFAIK , tracks that are written for it sound awesome because of the extra fx that get applied. If you are into hearing midis or creating your own, needless to say the pro is better.

I would get that 60$ 88 even if it was only to use as a 55 in games, it's just such a good deal to pass it IMHO.

The SC-88 does provide extra polyphony versus the SC-55mkII (64 voices vs. 28), so it may be of use in some OSTs.

Being an SC-8850 owner, I'd be inclined to say that for most games SC-55 and SC-88 maps are more than plenty. There are cases (mostly from Japanese developers) where the 88Pro or even higher are required, but that is not often the case.