Here is the problem based on my limited understanding of this whole business. Just as important as the sound module is the quality of the midi file itself. Midi is programed, so,

Bad Midi programing + Expensive Sound Module = Bad sound
Bad Midi programing + Cheap Sound Module = Bad Sound

OK, so you see where I am going..

Good Midi programing + Cheap Sound Module = Bad Sound
Good Midi programing + Expensive Sound Module = Good Sound

Actually, I don't even see where this is going...

Oh, OK, now I remember, so unless you can contol the programing of your MIDI, there is no sense on spending a lot on the sound module. Go with the $40 ForteDXi or the Cakewalk TTS-1.