I've come to the conclusion that there's little to choose between the motors. The 250w Bafang is the work-horse. The Q100 is better for light weight bikes, but needs 17 amps to get the best out of it. The Tongxin is supposed to be the quietest, but the rollers can slip under high torque, and there's some questions about its robustness. I haven't tries the MXUS because for me (UK), they're not easy to get.
For higher power, my present favourite at 36v is the 500w Bafang CST. It's wound for 270 rpm, which means it can hold 20 to 22mph if you want. It has exactly the same speed and power characteristics as my 500w BPM (code 1, I think). I prefer the CST only for its cassette gears, but it costs a lot extra for that small gain. Inside the housing, Bafang have improved a lot of things. It's a real quality motor now, but having said that, I never had a problem with a BPM.
2-wheel-drive is better for slow riding on difficult surfaces like snow, mud and wet grass. You get the advantage of being able to use one motor for efficiency when the road's flat, and then you also get good efficiency with two motors when climbing. Apart from the better grip and balance, there's not really enough advantage to beat the simplicity of a single rear 500w motor. If you already had a single front 250w motor with a suitable (for 2 motors) battery, it might make sense to add an additional rear as an upgrade.
For 2WD, you need two controllers, two throttles (or one double throttle - see threads) and one battery capable of giving at least 30 amps.You could use your main motor on PAS, and have the second motor on a single throttle for boost. This would work well if you had a controller with variable PAS like the KU65 or one with an LCD panel like those from Conhismotor.
You won't be able to swap between powered and unpowered bikes at the flick of a quick release lever. Most of these small motors have solid 10x12mm axles, so are bolted on with nuts. Depending on your forks, you might want a torque arm as well. Plus you have to disconnect the wires, which can be complicated if you don't get a connector near the hub, although most now have them, but check with the supplier if that's important for you.
Where to get them from depends on the risk you want to take. The advantage of buying from Chinese companies like BMSBattery is that you get a large choice of batteries. When you buy the cheaper kits locally, they tend to have batteries of limited power, which means that it's not so easy to get decent climbing power, which can be important if you weigh 90kg or more, but not so important if you're a 50kg waif. The Chinese seem to have improved a lot during the last couple of years, but warranty isn't practical because of the high cost of sending stuff back, but then you get the stuff so much cheaper.
