Which is the better plan?

IMO I think you should strike while the irons hot! If you are currently doing well accademically in high school, then apply to Columbia and NYU for your undergard degree. Then, if you still want to pusrsue creative writing, go to Iowa for grad school. Plus, keep in mind that while in your undergrad, you may want to get your masters in something totally different---many students end up changing their majors at least twice (I did)!