150 megs vs. 500 megs. You should get better quality from the 500 version (more samples per octave, perhaps), but likely greater polyphony from 150. I\'ve got GM500. Wish they had included GM150 with it, as I would love to mix and match between the two but can\'t justify purchasing both.
I also bought the GM500 CD. Frankly, it is a very mixed bag. Some sounds are great, eg. nylon gtr. Others are poor, eg. Saxes. I can get much better results from my Korg Triton when playing midifiles. The multiple fx units in the Triton have a lot to do with this. Anyway, JMHO.