#2 for me. Start a Project in Cubasis and export to Cubase for finishing. Or if you have something in Cubase that needs more playing around with to improve, just Bounce it and export to cubasis, now you have a ton of wicked iPad synths to mess around with. Export the results (midi / audio / …) back to Cubase and finish the track.
I also use it a bit like #3, the pads and keys and such. Steinberg should add the cubase quickcontrols either to cubasis, or to cubase ic pro along with keys and pads from cubasis