The difference between the INrunner and the OUTrunner is easy.
Without techno-babble, the OUTrunner does this: the outer case (aka can) spins. The prop/ adaptor is bolted on the case.
On the INrunner, the shaft going through the middle spins. The outer case stands still.

The difference this makes is that the OUTrunner is swinging those magnets (inside the case) around. The weight of them spinning creates a lot more torque. Since you have a lot of torque, you don't need a gearbox to spin the prop - it can be bolted directly to the spinning case.

On the INrunner, you almost always have to use a gearbox to gear down the motor, thus creating the torque needed to spin the prop. Gearbox=unnecessary complications, noise, breakage.

Hope this helps,
Bil