The Mana Sword posted...
Mario is more consistently good.
Zelda has higher highs and lower lows.
I feel the exact opposite, honestly. (Assuming we're looking at main-line series). Mario has some incredible standouts. Zelda has a bunch of solid games that are worth playing through once.
I care more about standouts, though, so...
Mario
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Cats land on their feet. Toast lands peanut butter side down. A cat with toast strapped to its back will hover above the ground in a state of quantum indecision