As the topic says, which motor (with same power rating) is to prefer for a standard (3kg) sumo?
I think I've read somewhere that it's harder (takes longer) to stop a motor if it has a high voltage rating. True?
Or are they basically the same?
I always look at the RPM (speed) and torque. In this case, for the same RPM you can get a lot more torque at 12V vs 7.2V.
In my experience, motors in the 12V - 20V range can stop on a dime. I would not call 12V "high voltage" anyway.
So I'd say it's a no-brainer unless you don't have room for batteries. With NiMH you would need two 5-cell packs in series to make 12V. I would probably use a 3S lipo pack if I wanted to run a 12V motor.