If it were me...I would let go of the reins! I don't care in that moment if my horse is going to run off the cliff of the grand canyon, in an emergency situation take care of yourself first! (And horses are perfectly designed to take care of themselves first anyway. If they are truly scared/p*ssed, they will make no special exceptions for their rider) It may be sad, but you can replace a horse, you can't replace your ability to walk if you are paralyzied. And you definitely can't replace yourself if you die. And then you can't ride ever again.
And also, if you hold on, you can't get away from the horse. And when you are hanging on the side of the horse, you are almost garunteed of getting kicked, drug or trampled. I would much rather deal with a runaway/bucking horse on the ground, OUT OF KICKING DISTANCE.