Benny, you seem to have a good grasp on what's going on here. If your budget can stand it, I would always go for the bigger battery. Over 3-4 years battery degredation can rob you of a significant range capacity. I can almost guarentee that you will not be using this bike for just your commute, mainly because they're such a blast to ride, so the extra range will come in handy. Also it could be charged just a couple of times a week.
Motor max rpm is achieved at magnetic saturation, this will be it's no load speed. Maximum bang for your buck seems to be when the motor is run at around 80% of this max rpm (but 50-80 is OK). So a motor that gives a 17-18mph reading on the stand, run at about 14-15mph would be the best efficiency(hub motor). Running this down to 8-9 mph will not produce significant heat at the motor. I find I get up most hills at around this speed.
The 250W rating is not the motors consumption, this is dependant on the controller amperage rating. The motor will consume whatever the 'valve' of the controller allows through. Hence, on hills, 6-700 watts is perfectly feasable (in max setting), dropping to much lower figures on the flat as speed rises. As it rises towards magnetic saturation, the back emf generated limits the current drawn. Hence riding at or near max rpm less power is drawn.
Well that's the theory anyway, start adding if air resistance, wind, weight etc and it's a maths nightmare - well beyond my little brain to figure.
