How many days off do you have, and are you ready for complete improvisation or do you need a solid plan?
If you're short on time (a week or two), a tour is actually a smart move - you'll save time on organizing and won't risk getting lost between settlements. The roads there are rough, transportation is scarce, and if you're trying to catch a ride on your own, you might spend more time waiting than actually seeing the volcanoes. Though with a tour, you're stuck to the route and the group's schedule.
But if you've got around a month and you like figuring things out as you go, you can rent a car - the roads are accessible, the scenery will be all yours, and you can stop wherever you want. Just keep in mind that gas is expensive, it's harder to find a car during peak season, and I wouldn't advise going out to remote areas alone without cell service and supplies.