I have Nintendo Land, Mario Bros U and Batman, and would choose Batman out of these, but since you only ask between Nintendo Land and Mario, I have to ask what kind of game you're looking for?
Nintendo Land is very fun, many different small games to pick up and play, they're all easy to learn but hard to master and it has somewhat a classic arcade feel to them with high scores and all that.
Mario is a longer game, but it's much of the same. Classic side scrolling Mario, a couple of new abilities / items, not that many secrets...