I own the MC-80EX, as far as I remember there are no difference between the two, but the sound board. I used to enjoy listening to a lot of midi files made for the SC88 Pro, since I love early Roland sound character. But for editing and sequencing it's not too intuitive, since we need to use Microscope edit just for selecting patch and other editing. Get the EX only if you like older Roland Rompler, planned to dig the Sound Canvas world and ready for an extra effort in using the engine. There are still many people who are enjoying music made by early Sound Canvas, specially older games music. The MC-80EX is certainly the best hardware midi player for that. Some people also using the sound board for a metronome use. But if you didn't bother and only going to use it as a hardware sequencer, the standard version would be enough. Look also for the RM1x/RS7000/QY700, which are more interactive and has a much better voice editing and mixer. But I like the SC88 Pro sound better.
With something like the new
Virtual Sound Canvas, which has a SC88 Pro sound map. It can be useful as a hardware midi player, using midi files created on the PC. 15 years ago, when laptop laptop were still very pricey, this could be a dream come true. It almost like a PC editor for the sound board.