I've been using Multitrack Studio on my iPad for a few weeks now and I find it pretty good but it needs a few things. Mostly in the score editor. But it is probably not geared towards score writing.
Just the damn Piano Roll. (not a Fan) There are control handles in the Piano Roll that are not in the Score editor. Too bad. But it is still better than Symphony Pro those guys can't seem to get their finger out. That also is too bad. I still find myself going to Music Studio for realtime recording as that is also lacking features in Multitrack Studio. Well so much for that.