MarioKart 64 or Double-Dash
I'm curious, why did you choose these two Mario Kart games instead of something more recent? I think that Mario Kart Wii or 7 would be a better introduction to the series/videogames because the control in them is way, way better than older games in the series. The general mechanics are better as well, and in the case of Mario Kart 64, the graphics are too primitive.
Likely for the same reason I chose the games I did, the early games in the series set the precedent no matter how much better the latter games get the originals are where we started and thus think others should start too. I would recommend everyone play Pac-Man even though MS. Pac-Man is generally accepted as the better game, it's because you want to experience gaming in it's entirety not just the current stuff.
Look at it this way, if someone plays MK64 or DD and like those then they are more likely to really like the better newer ones, but if they start with a newer one and go back to one of the older ones they might not like them at all or as much and thus miss out on some great games from the past.