Just a summary of my teams experience this year:
Early calculations showed that we would achieve higher speeds with the smaller motors and wheels that would fit the dimensions the team members desired. Physical tests confirmed that this was the case and that the small motors were very ineffective with sliders. This would free up the larger motors for beefier task.
However, as the robot evolved and the team added and reinforced features, the weight of the robot increased – more than anticipated. With the weight of a loaded mine cart, the small motors were no longer able to reliably navigate the tunnel lips. The drivers had to rock the robot back and forth to eventually get through. It drastically impaired our performance.
This is leading the team to consider modifications before the regional. Specifically, they are considering switching the motors and adjusting any corresponding gear ratios as well as reducing weight.
One additional complication was that the repeated sudden direction changes led to the gears stripping in one of the motors. The bright side of this is that the team now knows how to repair the gearbox and the importance of programming a damper to reduce the impact of sudden direction changes.
A different item of concern with the small motors is simply their ruggedness. While not the best practice, it is common to have wheels mounted directly on the motor shafts. (We may move away from this.) This puts a lot of lateral force on the motor shaft. Previously, we had never noticed an issue with this, but with our bulky robot and continued impacts on the tunnel lips, our drive shafts have started to have some lateral play. We even had the thin screws that mount the gearbox onto the motor break. This is definitely not the way a motor should be treated and we have learned our lessons. The large motors have a stronger construction, but we are still going to treat them more respectfully
In summary… unless you have a very light robot, large motors are probably the best choice for drive motors.