I dont see how either is stronger than the other, it is more a matter of aesthetics. The miter looks cleaner, especially if the frame is made out of one continous piece of wood, and is harder to do without a gap. They both use end grain, which doesnt glue up as well, but the butt is at least using on side grain.
What really beefs up the joint is the glue block on the inside of the frame. But, at least to me, the corner joint doesnt take away from the rest of the accordion, especially with the corner protectors on. I just boils down to taste, and it seems silly for this to come up so much.