rental
I would personally NOT use this type of service, but for Apple to offer both, is a good idea, it gives people a choice. However, I sincerely hope that we are not ending up in situations with "rental only" titles, and vice versa, I would realy hate that with a passion.
Alternatively, I am not a big TV head, so movie rentals, and music purchases as a combination may work for me. Just let me buy my music, I don't want to rent that! Movies, I rarely watch them more than once or twice, music I listen to a lot, and several songs over and over.