Leo's RC Simulator is basically a sandbox for playing in as there are no goals or end game to strive towards, it is all about playing around with RC's in the sky, which for those interested can be an endless joy.
Quadcopter FX Simulator Pro is a full upfront paid app that costs $4.99 and has zero in app purchases. Meaning, once the app is paid for there will be no other transactions, which for this particular genre is refreshing to see.