They are both really fun to have. I think ATV's are more fun and safer.
On the dirt bike you have to have pretty good balance and it's easy to fall off and get hurt. For ATV's you don't have to worry about balance you can go more places with ATVs for example you cant really cross a shallow river with a dirt bike but with an ATV you can and its a lot of fun. You also don't need to buy the expensive equipment you need for dirt bike riding if you have a ATV.
In my opinion ATVs are more fun for off roading and mudding and they are safer.