ATV by far, I've never rode a dirt bike, but when I had my ATV, it was so fun going out to El Mirage in So. Cal and riding all around and it was fun just because of the rush you get the open ground before you and dust clouds behind.
I never rode a dirt bike but when I rode an ATV I felt awesome. I know a friend that has both of them and he says a dirt bike is better. But if I want safety, I would take the ATV.