They all come from China and they are all pretty much the same. Some have cheaper (is it possible?) ancillary parts like the clutch lever and chain tensioner.
Here's some tips to get you going toward a reliable bike, the more you do the better and it will pay off in the long run.
Bicycle Motor Preperation And Care
I've made countless rides of 50 miles or more once the bicycle motor kit was broken in.
Also, there is a thread around here somewhere that list all the venders....so far. Some are better than others if you have a problem or warranty claim (thatsdax for one)
Some will not return your emails or answer the phone, and some will just blow you off...revolutioncycles .com for one. Boygofast is somewhere in the middle.