I'll come out of lurking just to clear stuff up. As Shou stated, when it comes to X68K and MIDI you'll be hard pressed find someone with more experience them me.
First off "GM Modules" doesn't really apply here. Street Fighter II', Super Street Fighter 2, Daimakaimura, Asuka 120% are the only GM compatible games I'm aware of. Dracula has a hidden "GM mode" (push XF1+G on the MIDI select screen), but it's very buggy, and doesn't work right even on my TG100 (pure GM only module).
The majority of X68k games use LA (MT-32) or GS (SC-55) formats. LA and GS hardware is completely different. GS modules like SC-55 are essentially a bunch of rom samples, they can be edited a bit, but that's all. LA however is a real synth, meaning totally new sounds can be created by the programmer. When a GS module (all Sound Canvas units) is set to "MT-32/CM-64" mode, it just remaps the instruments to match up. It does not understand system exclusive commands, and can not create custom sounds like the LA module will. So basically a game like Phalanx (MT-32) will sound ok on an SC-55 as it only uses the basic from the factory preset sounds. However a game like Dracula that uses totally custom patches in LA mode will sound like shit. And while Phalanx will sound ok, it sounds better and more natural on a real LA based device. Short story, play LA based games on a real LA synth.
For LA modules there is the MT-32, CM-32L, CM-64, and CM-500. MT-32 is the core, the 32L adds 33 extra sounds, the 64 is a combination of 32L+32P (pcm based module) in one, and the 500 is a cm-32L and cm-300 (sc-55) in one module. Some games only use the MT-32 base, while others use the extras of the 32L, and a few use the full on 64. So if you run a 64 based game on an MT-32, it will work, but you'll be missing all the extra stuff.
The 500 has a very good CM-64 simulation mode (uses GS hardware to make up for the missing 32P of the real 64). It sounds quite good, but when directly compared the real 64 sounds a lot cleaner. Also the real 64 is the only module which offers the PCM Expansion cards (this is a CM-32P feature). A few games actually use these, and only the real 64 can do this.
Examples:
Gemini Wing:
http://www.youtube.com/watch?v=yZq9VIVF6Vg
Choujin:
http://www.youtube.com/watch?v=Z4jq3ql8etY
So for me, I say get a CM-64. As it will play all LA based games to their fullest and there is no replacement.
Now moving on to GS based games. They were all written of the SC-55, but the 55 is old and noisy. Using a 55 is the "pure" approach, but a later GS synth can be a lot cleaner. I've experimented a lot, and for me the winner is the Yamaha MU2000 EX or MU1000 EX. Both have the same sound hardware, I choose the 2000 as it has some really kickass features like a hardware sequencer (it records and play back midi files in real time totally by itself)
Of course when using a high-end module like the 2000, your result is also depends on your audio gear. On the 1000/2000 the digital TOSLINK audio output should always be used. And the quality of your DAC, amp, and speakers will make a huge difference.
Also be aware for the GM games, the SC-55 kinda sucks imo. The SC-55 technically isn't GM compatible (even though it claims to be), even my cheapo TG100 (only 12bit sound) kicks it's ass when used with Daimakaimura
Summing up:
Budget setup: CM-64 + SC-55
Quality setup: CM-64 and the best GS unit you can afford
Baller setup: CM-64 + MU2000 EX plus high-level audio gear.
The Yamaha has some crazy feature to play with too, but it gets expensive:
X68k SSFII Ken's theme with a a drum plug in board:
http://www.youtube.com/watch?v=Jw9yFxzNniU
Finally here's my MIDI Parade vids. All done using 100% real hardware. Note these are LONG vids:
MU2000EX:
http://www.youtube.com/watch?v=ZsAQQzuSmpw
CM-64:
http://www.youtube.com/watch?v=VhTYhlkJTTI
My MIDI rig: