Horseback riding is BY FAR more dangerous, even though the surface on which you fall is more forgiving. I ride horses myself and can testify to the danger of riding 1,700 pounds of muscle and testosterone over a four foot jump at 25 miles an hour is increddibly dangerous.
I was riding cross country (an event where you ride heck bent for leather across uneven, often slippery terrain and jump solid jumps at a gallop!) and my horse spooked, and galloped at an ungodly speed towards a jump, but didn't jump it. He slammed into it chest first, and then flipped over the log. He broke his neck and had to be put down, I had a hole in my skull the size of a baseball. I'm riding again (much to my doctors chagrin.)
Horses:
-Horses have wills of their own
-Horses can (and do) say no
-They have blind spots, thus mking them unpredictable, even in the most farmiliar situation.
-They are aware of the power they posses
Motorcycles (and why they aren't dangerous):
-they cannot think for themselves
-you can control them
-they are not pumped full of testoserone
-they cannot, kick, bite, buck, rear or spook
So yes, riding is much more dangerous. I know firsthand how dangerous it is! Horseback riders are also tougher and stronger than motorcycle riders!