Dragon Slayer's a better game, but Dragon's Lair is definitely more "dragony".
Playing through Dragon Slayer II right now (about 2/3 of the way through). Not too impressed with this one. Game progresses way too slowly, and even when you characters are properly equipped and leveled up, a weak peon takes 8-10 hits before you kill it. It puts way too much emphasis on level raising & money collecting. As average it is, Part I is still a much better game.