
Hub drive motors are mounted inside the hub in one of the wheels. There are font wheel designs and rear wheel designs, but the ups and downs are pretty much the same. You'll see this type of motor on bicycles that sell for under $2,000. Bikes like these are great for short commutes, weekend rides with the kids, or gettin' groceries. If you ride mainly on flat ground at high speed (you probably don't live in Seattle), this is where a hub motor is most effective.
Mid drive motors are mounted at the bottom bracket. They provide a lot more power and efficiency, but are not a design that can be taken off easily (or at all on most designs) to convert the bicycle back into a regular bike. If you choose this system, your electric assist bicycle will be a full-time electric assist. You'll find most mid drive systems at $3,000 or more. If your commute is long, or the rides you want to do are long with hills, choose a mid drive unit with lots of power and a good battery.