I don't know the M-DEC but know a litle bit about the G-DEC
Be aware that MIDI is only data and NOT music, basicaly it is just a stream containing instructions like 'Note D on, Note OFF,....' so what you hear will only depend on the soundmodule in that M-DEC and won't change with another midi module.
I would be reluctant to buy a hardware midi-player for live gigs. Don't forget that the more you have on a live gig the more can fail on a bad moment ie. in the midle of your gig.
You could consider a couple of alternatives... why use midi on stage? prepare you backing tracks as MP3 and put them on a i-pod or equivalent ?
If you have some money to spend have a look at the soniccell from Roland :
http://www.roland.com/products/en/SonicCell/index.html , a brand new concept which looks very promissing.
Hope this helps, don't hesistate to ask other questions
My golden rule for live gigs :
Keep it as simple as possiblek