It depends on the rider.
If the rider is equally skilled between both machines, usually the bike is safer, simply because it is easier to avoid stuff on a bike. Bikes can go places quads cant (as easily at least, and sometimes quads cant go where a bike can go), and some trail stuff can be avoided on a bike the quad HAS to go through.
However, if you wreck on a bike, you have the inherent nature of the bike to go over, where as the quad will want to stay on its wheels, unless you take it airborn. But it may be easier to bail from the bike and the bike is USUALLY lighter than the quad so that can be a factor as well.
I dont think its really much of a fair comparison as it depends on the circumstance and the rider more than the difference between the 2 machines.