If you set both motors (1512 and C50L) up so the car drives basically the same around the track and putting out the same power then there shouldn't be too much difference in runtime, probably not even enough to tell if it's the motor or inconsistencies in your driving. The only difference should be down to motor efficiency which I would say is slightly better on the neu, but it won't be huge.
Motors do draw watts, they draw watts to make watts.
Efficiency is the % they put out compared to what they use.
If you have a 2 pole motor and a 4 pole motor of the same size and kv, the 4 pole motor isn't going to have twice the torque of the 2 poler, because to be the same kv as the 2 pole motor it has to have a lot less turns, and lower turn motors have less torque. If you do have twice the torque from the 4 pole motor it will be half the kv, so in the end it will basically even out. You don't get things like double the torque without sacrificing something else (kv in this case).