It seems obvious for me that it's Mickey. Except maybe in a family of (Nintendo) gamers, but in a "normal" family the kids will watch Disney movies, want to go to Disney World/Land and get Mickey or Minnie plushies long before they even see Mario.
My brother in law is even a huge gamer but not a Nintendo one, my nephew and niece are huge fans of Mickey and Minnie (and Nemo, Simba, etc...) but never even heard of Mario. It could depend on the country too, I don't know, but I can't see Mario anywhere outside of the relatively small gaming world (compared to the Disney one). You're wrong if you think that Mickey or Minnie aren't in anything Disney is doing, they are everywhere in merchandising and contents made for kids : books, cartoons, plushies, school supplies, toys, even clothes...
And if we talk about the other side of the age chart, no doubt grandparents will know Mickey but not Mario (again, except the few who have been initiated to video games by their kids or grandkids). So overall, it's surely Mickey.
It looks like a question relevant only on a gaming forum, I bet a lot of other people would laugh at this debate ^^.