I always shoot 720 24P Native when I am concerned about hard drive space, such as for long form projects. With a 16GB P2 card you will get 16min of recording time in shooting 1080i60, 1080p30n, 1080p24n, or 720p60. You will get 32min shooting 720p30 (no audio is recorded because of the frame rate), but 40min shooting 720p24n. 24p native gives the most recorded time because there is not any duplicate frames to be pulled down. Since you are dealing with less frames, it also takes up less space when transcoded to QuickTime and less time to compress to DVD or web.
But as others have already answered, the best thing to do is TEST the different shooting formats and see what works best for your project. Each format behaves differently regarding light needs, motion blur and strobing, etc.
