BionX 250 and 350 are both nominal wattage ratings. Those who have ridden them know that they give much more power when climbing hills. I have measured well above 1000 watts with a Watt's Up meter spliced between the battery and motor of my BionX 350 when pedaling hard on a reasonably steep hill.
Will an SF dealer let your son try the 250 and 350 on his route? Perhaps he will want to ride steeper hills in other parts of the city, though. In any case, test riding is critical to making a decision.
A torque sensor is far superior to a cadence sensor for anyone who is an avid cyclist, especially on hills. Don't even consider a cadence sensor bike until your son has ridden both enough to know the difference.
Here is a BIonX 350 on sale for the price of a 250, $1199. I haven't read the small print, is likely last year's model with a freewheel instead of cassette, possibly other minor changes, but may be a very good deal.
http://www.biketechshop.com/bionx-pl-35 ... -2410.html
The older I2C BionX kits (about five years old or more) could be powered with higher voltages. My PL250 has run at 36V and my PL350 at 48V.
Search craigslist, ebay, etc for any possible used BionX kits for sale, it happens from time to time.