I was steered to Grubee kits right off the bat, only used their Skyhawk motors, and bought them all from Motorized Bicycles - Bicycle Motors - Bicycle Engine Kits - Bike Motors - Bicycle Motor Parts
so I can't speak for the others, but I am very happy with the Skyhawk and gasbike does tend to go out of their way even for small guys like me.
I wanted 3 black Skyhawks, 2 48's and a 66 on my last order.
They didn't have anything black in stock as a kit, but they DID have the motors stand alone.
I asked if he could just swap the motors out on other kits.
Apparently nobody had ever asked that before.
He got his supervisors OK and did just that for the same price as the kits he used (actually saved $27!).
That is the kind of service I will stick with even if it is not the best price, which so far has never been the case anyway.