Mario Vs Zelda = Miyamto Vs Miyamoto then!!!! then my choice goes for both with little bit to hmmmm.... big huge thinkin'.... ta tah ZELDA of course.

Fixed

I think the two games are hard to compare. Like many people said, they are not aimed at the same audience (though there is of course a huge overlap). The themes are different, as well as the gameplay. Mario (I'm only talking about the 3D Mario games like 64 and Galaxy here) is better suited for short sessions because it is divided into stars, is better suited for multiple audiences and makes for an exceedingly fun gaming experience. Zelda however features an actual storyline (unless you wanna call the repeated kidnapping of Princess Peach a story), weapons like sword and bow which appeal to many gamers, and a more serious and beautiful world, both in terms of graphics and storylines.
However, if you compare both series' latest offerings, you'll see that Mario pays much more attention to a completely new gaming experience, whereas the main additions to Zelda were intuitive sword control and bow-aiming by hand. Zelda keeps offering more of the same, which is of course fine because it has always been great, but Mario actually wowed me with its new features (gravity, planets, 2D/3D switching, Ice Mario etc.). Still, I had an equal amount of fun with both, so I agree with Nintendo Fan Girl.

This is a really hard one, but I'll go with Mario for its simplicity and fun.

i vote pokemon