I use FL Studios because I personally prefer the piano roll environment, as opposed to say, the sheet music composers. I don't really know about Pro Tools. But that's just me. I've composed plenty of songs in it just fine. However, if your comfortable with the piano roll and you enjoy it, I'd say stick with it and FL. A composer is better off working in an interface he/she is more comfortable and familiar with than one he/she is not.
As far as instruments are concerned, I don't really use the instruments that come with FL, rather I use soundfonts, samples, and dsl banks
ripped from DS games. However, it's not that I don't like some of the instruments in the pack; it's just my style and preference are more...orchestral, as opposed to the extremely synthy sound that most of those instruments have. Again, it's purely a matter of preference.
Old signature is old.