I've had the TD-3 for a long time and used it for the typical 303 sounds; aggressive accents sequences and for beautiful melodies leads and arps.
But I wanted to get more control and for that "Devil Fish" flexibility. So when I got the TD-3-MO the first day I was bit confused as some of the controls were not acting as they should, for example:
Overdrive: if I have it all the way down, no sound comes through (even having the Volume all the way up) so it always has some overdrive to it.
Decay (filter): if I have a short decay it sounds like it shortens the actual decay of the synth. Not like the TD-3 or original 303.
Then there is stuff like the Accent Sweep that I have to keep always at 2, if not it just sounds too resonant. The SUB OSC is okay, never really use though. And have tried to make some melodies like I do with the TD-3 it just didn't sound that good.
But the thing I use it for, and why I won't sell it (for now), it's for those aggressive Accents sequences, anything that needs a more aggressive 303 I will use the TD-3-MO and being able to control the decay independently of both Accent notes and Normal notes adds lots to the groove
So in conclusion, if you're looking for an overall good sounding 303 go for the normal TD-3. But if you want that more aggressive sounding 303 the TD-3-MO is the way to go.