Definitely thights.
stockings are suppesed to be worn with skirt/dress. Even if the stokings' tops appear occasionally under the hem, you can pretend it's "accidential".
On the other hand, there is a difference between regular
stockings and thigh-socks, without laces. Thigh-socks SHOULD be all visible, so maybe it's OK?
Whatever you choose, don't forget smart high heelsed pumps!