There's also gearing to consider. A motor with a given voltage and current can be geared up to increase speed, with a reduction in torque, or it can be geared down to increase torque, with a reduction in speed.
You have to decide whether torque or speed is more important. And, of course, which is a limiting factor. A 40000 RPM router motor is going to be next to useless if connected directly to the wheels of your bot.