The differences in behaviour between rm, mm, 3 and 4 gear are all on the site.
The main differences are in how it behaves under acceleration, cornering and in the air. (Balance and traction)
http://www.team-durango.com/in-the-p...rticle_id=4472
FWIW the "main" versions used are mm4 and rm3. Some use the other options, but not (afaik) that many.
And you don't add a gear, you take one large idler out and put two smaller ones in instead. No change to final ratio. But you do change the direction of rotation of the motor in relation to the wheels.
BTW in all configurations the motor runs "forwards" it's just the arrangement of the spur, motor and gearbox that change.
I did comment on another thread (same subject) a week or so back, have a search