d8veh you soud like youve had some experience the Xiongda and though that kit doesnt suit me and my bike it seems to have been okay for you.
Have you had chance to compare the 350w Xiongda to a 36v 500w BBS02? On paper 350w should be inferior to any 500w system when moving at speed.
I think you've over-simplified it. There's a lot more to a riding experience than power. If you can't pedal very hard, you need a motor with good torque. Only if you need to sustain a high speed do you need high power. I have to agree with Tom that 14 1/2 stone is not that heavy, so a larger "250w" hub-motor like the Bafang BPM, Ezee or other such motors should be able to get you up very steep hills without to much effort from you.
Hub-motors give a much more relaxed riding experience because you don't have to worry about gears so much. It's nothing to do with being lazy, more that crank-drives are a bit frenetic.
Don't get too hung up on power ratings. 250w, 350w, 500w, etc doesn't mean much because hub-motor winding speed can make a big difference to how powerful it feels and, obviously, how fast it goes.
To answer your question, I have compared those two motors. It's the power that you run them at rather than the power rating that decides how powerful they are and whether they are running in their efficient zone. In theory, the crank-drive should always be more efficient, but my tests have shown that that advantage doesn't get realised in practice, however, having the wrong speed winding on a hub-motor can destroy your efficiency, so you have to choose carefully.
There's a very nice kit from BMSBattery that uses the 48v 09 bottle battery with the included 20A sinewave controller. The wiring is therefore as simple as a BBS01. Add a Q128, BPM or Bafang CST and you'd have a system that I guarantee would satisfy your power and torque requirements, whilst giving you a much more comfortable rode than any crank-drive.