With a Reloadable motor (For example, an Aerotech RMS motor), you need to buy the case and closures, then you can reload it with a wide variety of different propellants. For 'G' size, perhaps $5-$10 cheaper for each shot, once you have the case, and much more flexible in terms of the thrust, impulse, and propellant types available. It's kind of neat to be able to build up a motor from component parts; one way to do that without investing in a case is buying an Aerotech LMS motor - they get built up like an RMS motor, but the case is only designed for one use.
With a Disposable motor (For example, the Aerotech DMS or Econojet motors), everything comes together in one baggy. You slide the motor into your rocket, insert the included igniter, stand way back, and push the button. When you recover the rocket, you pull the motor out and dispose of it. A lot more convenient, a bit more expensive and less flexible.
Personally, I'm a fan of the Aerotech DMS or Econojet disposable motors; but I simply don't fly that much, so they're very convenient for me.
