30 is better than 18
Not necessarily. Overvolting a 12v motor to 18v is a 50% increase, overvolting a 24v motor to 30v is only a 25% increase. When you double the voltage on a motor you get 4 times the power out of it. So it's a % increase that you're after. This is why motors burn out, they produce more power than they can handle, draw too much current, overheat and burn out.

This makes the assumption that the two motors are of the same power output to start with.