I felt like chipping in.
Get Mario 3D Land. One of the best Mario games of all time, heck one of the best platformers of all time. Game really is a 3DS system seller. Layton is a great game, but as an above poster said, Mario will keep its resale value more, but this is also why you should get it for the free download. It will cost you more to buy a separate (most likely physical) copy of the game. Layton you should be able to get relative cheaper later down the line, since its resale value would be lower, so the cost it will require for you to buy it will also be lower. To save money, get the game that is most valuable, cause the other you can get cheap. Reverse logic. LOL. This of course is if you keep your games. If most games you play, then resell, reverse this. But I see myself going back to play Mario 3D Land years from now, so its a game I would not sell, like a Mario 64.
No bad decision here, but I do really recommend Mario. Good luck, enjoy the games.