If you want to do any cold-weather riding, then an ATV is a much better choice because it gets more traction and is more stable on flat ground.
Keep in mind, however, that riding in the winter is COLD.
In fact, even helmet usage is far lower among ATV riders than dirt bikers; however, even if you take the lower percentage of helmet use on ATVs into account, ATVs are still far more deadly in a crash than dirt bikes.
While ATVs may feel more stable, they flip frequently.
There are good reasons to choose dirt bikes, and other good reasons for choosing an ATV for your off-roading toy.
One is not clearly better than the other because it depends on what you want to get from the sport.
If you hop on an ATV with 5 minutes of instruction and just want to go for a ride, you’ll do just fine.
On a dirt bike, you’ll need more instruction and it’ll take you a few days before you really feel like you can comfortably ride without thinking about the bike.
However, hopefully this post will point you in the right direction.
In general, I recommend dirt bikes to anyone who wants a thrill ride that is easier to transport and more fun to master long-term.