It's not only extended vs bent, it's also down vs out.
Elbows out makes less muzzle flip compared to elbows down (according to my observation). Because if the elbows are at the outside, they can work better as shock absorbers. With elbows at the outside, the hands can move back easier. With elbows down, the hands can only move up (more precisely: if the elbows are down, then at the beginning of the recoil the hands are more forced to move up).
My elbows are quite bent and more at the outside than down.