A.J is right, if you never used a machine to polish then you should start with the PC.
I never used the Flex but heard from many that it has a lot of bite to it.
I use my rotary now only if really needed, the PC will do everything that I need it to do.
If I were you I'd stay away from the edges, the clear is really thin there, maybe you can practice on a junk car to see how to control the machine.
A friend of mine ask if he could use my rotary, I asked him if he used one before, he said yes, the outcome of his work was bad, really bad, the Flex can do as much damage as of the rotary, maybe not as fast but you can burn paint with it.