Piano roll's advantage is that when you're writing, you don't get caught up in all the fluff of notation, like dots and flags and sharps and flats and slurs, etc. It's a much more production oriented method of sequencing music. It's also much easier to copy/paste/transpose/offset with piano roll.
I also feel that piano roll is a better way to write because you get a better visual sense of the intervals between notes and the durations (e.g. a major third actually looks bigger than a minor third, a whole note is wider than a quarter note, etc.); this of course is a personal preference, and I have written a lot in staff notation before switching to FLStudio.
Remember, when picking a sequencer, what's the endgame? Do you want to sequence something up as a mock-up for what will eventually be played by actual instrumentalists? Or are you actually sitting down and creating the performance yourself? With the former, staff notation is best, but with the latter, I think that piano roll would be a better choice.