There are 4,000 motorcycle fatalities per 10 billion miles driven. Assuming an average travel speed of 30mph that works out to one fatality per 83,333 hours of travel. There are 1.2 general aviation fatalities per 100,000 flight hours. That works out to 1 fatality per 83,333 hours. Using those numbers the risk of death is exactly the same.
There is much room for variation though. Slower aircraft are involved in more fatalities per flight hour because they spend more time taking off, landing, and flying near other airport traffic. Take offs are the most dangerous part of flying because of full fuel, low speed, low altitude, and fewer options to set the airplane down into the wind on a runway.
Now include the actual number of crashes and injuries.
Start with the 88,000 reported injuries for motorcycles, not to mention the number of unreported injuries which are far greater than the reported, in 2007 as related to the estimated 12 billion miles ridden compared to the 1786 aircraft accidents in which only 334 were fatal killing 600 people. Then factor in the 660,000 or so private pilots compared to the number of motorcycle riders and the total number of airplanes versus registered motorcycles. If you want to use MPH it is only fair to do that for both vehicles.
You’ll find that motorcycle riding is more dangerous. When I ride my motorcycle to go flying I have to drive extremely defensively and have had a number of close calls with idiot drivers. When flying, it is extremely rare to have a mid-air collision. Most of all general aviation accidents are pilot related where as half the motorcycle accidents are unavoidable due to other drivers being stupid. I don’t have that problem when flying.