Both ATV's and Dirt Bikes are pretty dangerous especially if not used properly and you don't wear the proper gear and helmets.
As long as you wear a helmet on the Dirt Bike and ATV and you use the ATV or Dirt Bike as intended then they're pretty safe.
Even your car or truck is dangerous so I personally wouldn't worry because none of us make it out of the world alive anyway.
Make sure to wear the helmet and proper gear and you should be perfectly safe on either the ATV or dirt bike.