I have shown at both multiple times, but it's been about 5 years since I've gone to either, so my info may be slightly out of date :) I always preferred Showtime since it was put on by the Half Arab Association, and therefore, had more Half Arab classes. My mare is a Half Arab obviously :) Both were excellent shows though.
They were pretty similiar shows in terms of classes offered. Fall Classic tended to be slightly larger, both in individual class sizes and overall number of horses showing - or at least that's how it always seemed! Fall classic also tends to be one of the most competitive shows of the season. There was always a very high caliber of horses being shown, and most of the big name trainers in Michigan would be there. They also had an organist there, which is pretty uncommon at shows these days and was pretty neat. It was a well run show and was always an enjoyable time. Winning, or even just placing, at Fall Classic always felt just a little more special than placing well at the other shows!
As a side note, I prefer showing at Mason to showing at MSU - the fairgrounds in Mason is within walking distance of downtown, an ice cream shop, a nice park, and they have a great tack shop on site
If you have more specific questions, let me know and I will try to remember things well enough to answer them!