I visited the Mauna Loa Factory a few years ago and it was interesting, but I'm not sure I would recommend taking a cab just to visit it. There is a small gift shop, and a self-guided walk past viewing windows that look into the small factory. Probably no more than 30 minutes needed for the entire visit.
My recommendation, if time permits, would be to rent a car and drive to Volcanoes National Park (~30 miles from Hilo) and make a quick stop at the Mauna Loa Factory on the way back. Have a great trip!