Although i'm a big yamaha fan, i can't make up my mind about this... Why?
Yamaha -
32 poliphony, Roland -
64 (VERY Important)
Yamaha -
8 track sequencer, Roland -
16 track sequencer
Yamaha sounds -
70MB + GM, Roland GM2 ONLY (yamaha beats the crap out of roland in this part, even though roland has more presets, but still they are just GM2 sounds, not sampled sounds like on MM6).
Just for comparison... Korg TR (mid range workstation) has 64Mb of samples, Motif ES has 175Mb, and Motif XS has 355Mb
In other words, the MM6 sound much better but has lass poliphony which may lead to note drops (when you play a bigger arpeggio with a sustain you might loose the first notes as you play the higher ones)
So if you're looking for a good sounding keyboard and you wont use it extensively - go for the MM6