In the FL Studio Mobile HD current version you can add and remove bars in the middle of a song. Note lengths are also not restricted anymore. So some improvements have been done by Image Line. I find FL Studio Mobile to be my favorite sequencer in iPad. I also own NanoStudio, Beat Maker 2 and Music Studio. All of these have their strengths, but FL Studio Mobile is a good choice if you do modern electronic music and use it only to quickly sketch your ideas and then export the MIDI data to your computer to finish your songs. NanoStudio on the other hand has a decent VA synth and has better routing / chaining of effects and a decent mixer view. It can definitely be a better choice if you want to make a finished /polished song with your iPad. But if you're going to import the MIDI into your main DAW anyway, I can't really see any point spending time tweaking synth sounds with iPad.
Beat Maker 2 has better audio editing options and supports virtual midi. Music Studio is pretty much same as FL Studio Mobile. It has better acoustic instrument samples but lacks in modern synth, bass and drum samples. It also doesn't have drum sequencer but it has buttons/pads to trigger chords.