The 36v 500w has a bigger casing and (I'm guessing) the stator/rotor is heavier duty to facilitate the higher current of 25A as programmed at the factory. I have both the BBS01 250w and the BBS02 500w. I've reprogrammed both. The 500w motor is slightly heavier than the 250/350w motor. I can say that the BBS01 is a very capable unit, but if you are looking for speed over any other criteria, then you need more watts. (or pedal harder) Gearing is a factor, but there is no point in overgearing a 250w unit to get more speed, the motor just wont overcome the wind resistance. I'd go for gearing that allows you to a) climb the steepest hill you are likely to come across.............for me that's a 42t chainring and 34t biggest cassette sprocket. I then have a top gear of 42t chainring x11t smallest cassette sprocket. If I pedal at a comfortable 90rpm with my 700c x 35c wheels/tyres in top gear I can maintain a speed on level ground in still air of about 27mph using PAS 6-7. BUT that will restrict how far I can go. You could use the 46t standard chainring to get a higher top speed, but a 250w motor wont overcome the wind resistance like a 500w motor. The 500w motor given an identical situation will always be faster.
I tend to ride both motors in PAS 3-5. I recently did a 55 mile road ride over hilly roads on the BBS02 equipped bike and averaged just over 20mph, including riding through the middle of Gloucester and up some very steep hills. The battery voltage had dropped to about 34v after the 55miles. If I took my super light 7kilo road bike (no motor) on the same journey, I'd have averaged about 17.5 mph and I'd have been hot, sweaty and exhausted.
The 500w unit will you more in reserve..................and used sensibly its carried me over 80 miles on a single charge. Mine is reprogrammed to 16A max with 100% restrict speed, I just restrict current (progressively) in each PAS level.