Opinions vary, but rear wheel is generally better from a handling, stability and grip point of view. Unfortunately though, that rules out hub gears and usually means using multi-sprocket freewheels for a derailleur, rather than the superior cassette type sprocket set. Hence the persistence of front hub motors, for which the only advantage is two wheel traction, motor front, pedal power rear.
So it come down to what is important for you. If you are very fussy about derailleur gearsets or want a hub gear, it has to be front motor. If not, I'd advise go for a rear setup, but in truth the differences for road cycling are so small the difference is not critical.