theoretically.....
a 4 gear rear motor tranny should give you more traction, reason being is that with the 4 gear the rotation of the motor and etc. is rotating in such a way that it is "driving" the rear end of the car down/into the ground. (best for low traction situations)
the 3 gear setup actually goes the opposite way and, with out getting technical, creates "lift" at the backend..... (better for higher traction situations)
so it is a matter of traction.
but obvoiusly it may not be a huge deal as there are some pretty successful 3-gear cars
but a great tuning option.... I i ran a 210... I would run a 4 gear rear motor and tune the car to make it steer 100% of the time but thats my opinion