Cubase is like a halfway house so is better suited to live tracking than FL but if you never intend to record or edit multitrack audio then I see little point in using anything other than FL Studio. I'd compare Cubase to Logic but Cubase is more stable and not as irritating to use. Both Cubase and Logic have very nice plugins in the package - of a higher quality than FL Studio's and more job oriented; such as actually having a de-esser, rather than having to construct one with side-chain EQ, as within FL Studio.
If you're working mostly with MIDI, however, FL Studio is king and I still do my arranging and mixing in FL after editing with ProTools.