As a mtbr and a ww kayaker I can say that I think the chance of getting injured is far greater on a mtb. Hell, in five years of riding I've broken my leg twice, tore the hell out of my knee cartilage, and fractured my skull.

Never had a serious injury in 15 years of whitewater kayaking. But I did come this close to drowning...

So - chance of injury higher in mtb & chance of death higher in whitewater - that's my take.

Steve Z