I genuinley dont think either is more dangerous than the other, the common denominator is the rider; its all about attitude, as PP mentioned. The way the cycle or motorcycle is ridden play a key part. The past few motorcycle accidents amongst friends have been caued by car drivers changing position in the road without proper observation. I also think the level of minimum saftey equipment for motorcyclists should be raised; anyone caught riding in shorts and t-shirts should get a fine or something.
I do think though that both methods are alot more vulnerable, not just to other road users but to the amount of road furniture. Someone up there mentioned its not the fall but what you hit that hurts and I agree; I really think all unnecessary barriers etc need to go.