I knew that I wanted to build a multicopter and that the ardupilot mega 2.5 should be the platform for stabilization. At that point I did a lot of research work:
Tri-, quad-, hexa- or octocopter?
That qestion was quite easy. The project had to be cheap and simple, too.
So my first idea was the tricopter. It is cheaper than the other multicopters, because you only need 3 motors and 3 speed controllers instead of 4, 6 or even 8 each. But I thought, it would have been to complex for my first DIY project, because the motor has to be movable by a servo.
So, my decission was building a quadcopter. Only 4 motors and controllers, no moving parts except the props. This makes it an easy platform.