I know it looks kinda strange that I'm not as exhausted on an ATV than I am on a bike, but it's really the way I feel it. Maybe because an ATV is softer than a bike. I rode a friend's 400 yzf for 1/4 hour, when I climbed back on the banshee, I tought I was riding a jellyATV!
And of course, you can also fall from an ATV! It's not as stable as a bike on bumps, but if you move around enough, you'll stay on. With the bike, you better hang on to the handlebar and hold the bike between your legs...
I must admit though that I haven't ridden bikes and ATVs on the same trails...maybe the key of our diferent thoughts (most probably yes)?
Maybe because it's easier to rest on an ATV?

However, I still do have fun on my banshee, but I can't wait next year when I get a bike (like a 125 TM, or a 500 VOR, but there's a big price difference)

 

Wear a helmet, on a wheeler you may not ever dump it, however on a dirtbike you WILL dump it eventually. keep it in mind.