Have you tried Ableton Note? It's not a 1 to 1 version of Ableton, but it streamlines a lot of the features to make the equivalent of musical sketching app. You can then save your session and it opens perfectly in Ableton Live.

I'm not a huge fan of music apps on iOS, but I played with it briefly and it's quite good. They've really done a great job to make it a very musical tool within the limitations of an iPhone (and presumably iPad).