I don't see many using the Rimfire motors. But they interest me. I'd like to ask how you like them? So far I have only Hackers but plan on trying out the Rimfires just because I want to see how they perform or last.
As far as what motor to use, start with the light one and see how the system responds. Ie. how much heat is being generated. If heat isn't being produced above normal opperating temps then thats the best choice. Then if you need more power or some part is over heating switch to the larger motor.
An under powered motor will overheat the batteries as quick as trying to run too large of a motor.
There's a great thread going now called E power 101 at Flying Cir*us. Lost of good stuff getting posted there.