I haven't looked into mid-drives much. Hub-drives seem to be easier to install generally and are pretty much maintenance free as they don't wear the chain. I think a big advantage of the mid-drive is that the ability to change gears makes it less likely to overheat when going slow up steep hills. So a hub motor would probably be best for most commuters and mountain biking that is done at a higher speed (no steep hills to climb slowly), while a mid-drive would excel with steep hills. I would check Endless Sphere, though, as I may be completely wrong.
Helping the aerodynamics would be great, but I want to keep it as bicycle looking as possible.