The hub motor is the same size, rating, &c. Obviously there is a size difference between the two wheels for the gears, but the motor itself is the same.
Users I've heard recommend the front kit for most situations in order to have a good weight distribution around the bike. If you get a rear wheel kit almost all the weight (rider, battery pack, hub motor) comes down on the rear wheel. There is a slight price premium on the rear kit due to the additional cost of gears.
-Mike