Aloha Andrea,
Personally I prefer to be in control and have my own car. That way you can stop and see what you want to, when you want to, and at your own pace. Obviously many people look at it differently though since the tour buses are always full
You can probably rent a car for much cheaper than you'd pay for the tours, so if cost in an issue I'd go with the car rental if there are more than one person.
Here's the ultimate though...go visit Waimea Canyon by car and then take a helicopter tour and see it from above. You'll be amazed!
Go to this page and click on the link for "Air Tours"
http://myhawaiivacationpackage.com/kauai-activities.htmlI think that would be your best bet overall!