My husband and I are going to be in NZ for 7.5 days in February. We're looking at the guidebooks, and I think we're going to spend most of our trip on the South Island. Is this the best way to see what needs to be seen? There are so many amazing things on both islands, but people who have been there recommend sticking to one island for such a short trip.
for 7 days i would say only do the south island plenty for you to see for 7 days.
Yeah, doing both islands in a week is not really doable without spending far too much time in transit and not nearly enough time actually seeing the country. In a week on the South Island you can do a great little loop Christchurch, Queenstown, West Coast, Christchurch (or vice versa) which will encompass all the very most awesome bits of New Zealand (Lake Tekapo, Mt. Cook, the glaciers, maybe Milford Sound).
I suggested a more detailed itinerary for that loop in this old thread, which you might find useful.