I would like to know a bit about the weather, and any tours that take you to Hana? We will have a rental car, but is it better to take a tour...
You've posted in the "Kula" forum. You're not staying up there, are you? It will be cool, and you're so far from the beach (and the whales).
Thousands of people drive Road to Hana every year. You'd have the advantage of stopping where YOU want and even turning around if you get carsick. We don't advise driving beyond the end of the road; you turn around there and retrace your steps.
The weather in March can be iffy. It may rain, it may not!! No way of telling more than about 2 days out.
I've done both a tour and driven myself to Hana. Had a great time doing both!! It's mainly a personal thing whether you would like either.