Honestly, both should be great. Already clocked over 20 hours on Super Mario 3D Land (SM3DL) and with already File A and B completely beaten, it's getting a bit meh. Not sure if this is from playing too constantly, the game is too easy, or I'm just too hardcore.
Mario Kart 7 should deliver about the same equality content like SM3DL, but it's online feature is what takes the cake. I used to play PSP games a lot and the online experience would allow me to gain up to 200+ hours just because of playing online with friends. Hell I played Medal of Honor Heroes, a PSP FPS with online capabilities, for 3 years haha!
With the growing communities around the web with anticipation for MK7, this titles is getting BLOWN UP.
If you can play SM3DL for 1-2 months then I say go for it. If you had to pick one or the other, I suggest MK7 just because of its lasting appeal.