Quadcoptergarage.com just did a whole write up on this, which is great because its very detailed.

KK2.0 is a pretty good basic flight stabilizer/controller and only costs $20. But its kinda dumb, no GPS or automation. Which is where APM comes in.

Also, before buying anything Ardu APM related…consider using the new PX4 PIXHAWK gear instead. Its 32bits better.

Check it out: http://quadcoptergarage.com/upgrading-my-quadcopter-from-kk2-to-apm-2-5/