There are advantages & disadvantages for either, and much opinion here at ES. Rear wheels on bikes are designed to provide propulsion. given the dropouts and framing. Also a greater axle width. But also where the gears are located, so a bit more involved for mounting a hub motor. The front wheel is a bit easier to fully mount. But not designed as well for providing the motive force. More taking hits on the front wheel - look at the dropouts. You'll find kits for either, however, so take your pick.