The Legend of Zelda and Super Mario Bros may both be legendary fantasy adventure franchises that have been around since the earliest days of the NES, but beyond that are different enough that is a bit like comparing apples and oranges.
The Legend of Zelda isn't technically a RPG per-say, but is very much like a RPG with its magical medieval fantasy setting, combating monsters with the likes of swords and magic wands, massive top-down dungeon crawling, exhaustive arsenal of items to gather, complex puzzles to solve, plethora of hidden secrets, etc.
Super Mario Bros is a simpler, more straight-forward but still immensely fun action oriented side-scroller where for the most part the player proceeds to the left whilst collecting power ups, jumping up goombas, kicking koopa shells, etc. Certainly, it has its share of secrets and even a few puzzles--but nothing comparable to The Legend of Zelda.
Super Mario Bros is the perfect introductory game that even a very young child could pick up, understand, and enjoy with little difficulty. The Legend of Zelda is geared to a somewhat older audience, such as the teen who enjoys both Dungeons & Dragons and this then-newfangled NES.
For those old enough to enjoy both, it's a matter of what you're more in the mood for at the time: the simple-but-fun side-scroller or the more complex top-down dungeon crawler. At least that was the case for me back in the bad old NES days!