I don't think it really matters. The teacher's knowledge and ability to communicate it effectively are what's really important.
My son's a lefty who's had both. The right-hander helped him WAY more. Interestingly, my son seems to learn best by mimicing what he sees. Doing certain drills off the stretch, his right-handed teacher and he could face each other and he could become almost a mirror image of what he was watching...something he couldn't really do with a lefty teacher. The teacher got to watch him better that way, too.