Typically, motors with a lower kv rating have a higher torque than motors with a higher kv rating. A motor with a higher kv rating is more efficient at higher current levels than a motor with a lower kv rating. The least complicated reason being a higher kv motor has less turns - wire in the windings therefore less resistance. Like Matt said neither is " more powerfull " however each will be more efficient at a different current level for a given voltage. For example, if you reqire a power system to deliver 300 watts input power to the prop you could run a 3 lipo pack at about 27 amps ( 300/11.1 ) or you could run a 4 lipo pack at about 20 amps ( 300/14.8 ). After determining which battery system to use I would then choose a motor that has its maximum efficiency at either 20 or 27 amps at 14.8 or 11.1 volts then gear and prop it accordingly. Hypothetically, you would want to use the 5400kv motor with the 3 cell set up and the 4100 kv motor with the 4 cell set up. I would suggest that you try moto calc or similar program and experiment with different set ups until you find what you think is best for your application. This web site http://flydma.com/flydmahome.asp has a calc program on it that may help you as well.

S.