I'd say these two are probably the most recognizable video game characters ever. Between them who's more popular? I'd say Pikachu since Pokemon is big across multiple mediums including cartoons, toys, trading cards, movies, and so on. 

Same level basically. They're the only game characters on that pantheon of characters EVERYBODY recognizes, unless I'm forgetting someone.

Edit: Yes, I was. As someone below me pointed out, Pac-Man is also on that pantheon

Depends on what type of mushroom Mario had that day

Mario,
pikachu i assoiacte more with trading-cards, stickers, anime thats how it started in my childhood. The games came later i think

You said gaming.Pikachu is only recognized for the Pokemon cartoon and trading cards.no one knew Pikachu when the first few Pokemon games came out.So Mario is easily the bigger gaming icon and I don;t consider Pikachu an icon as it does not have it's own game like Mario.Pacman is the most recognizable gaming character ever BTW.Most none gamers I know recognize Pacman,but don't know Pikachu or Mario.

Mario because you said gaming.

Mario certainly

Ummm, in terms of gaming, Mario. Pikachu is obviously well known for being in the Pokemon game series, but he's also known for the anime and other stuff. Mario is exclusively known as a video game icon.

Mario, no question.

Technically, the games were released first (in Japan). But the west got the anime before the games, if I recall correctly.