From which rating would it be easier to transition to an Osprey? Fixed-wing or rotary-wing?
The Osprey is closer to a helicopter and if I was going to chose from a fixed wing pilot or rotary pilot to train on it I'd choose the rotary pilot.  The rotary wing physical skill set is so much more demanding than the fixed wing one.  A helicopter pilot could master fixed wing cruising flight in the Osprey far easier than a fixed wing pilot could master hovering the Osprey (you can pretty much train a monkey to fly an airplane in cruising flight - airplane physical demands are mostly when you slow down).