Partly it is an individual thing and partly a form thing. Form first: if you choose to have a fairly high anchor as an aiming aid, as for 'gun barreling', or if you want good short range gaps, you might try the high thumb method.
If you want to shoot more instinctively, or at long range targets, with a lower anchor, you might try the thumb down position. This one may enable you to tuck your thumb beneath your cheek bone.
But your physical structure may leave your little choice. Some archers can do one but simply cannot do the other with any smoothness and ease. Try both for a while........if you can. - lbg