It depends on the game. In Mario Kart Double Dash, Wario and Waluigi are a much more powerful pair. Mario and Luigi throw fire, which is much less efficient than throwing bombs, which both Wario and Waluigi are able to do.
Let's just get this straight. No offence Wario/Waluigi fans about the upcoming scene.
Wario is a fat lump who needs to cut down on McDonalds. Waluigi is a skinny twigshamo with moon-tipped shoes. Mario is an awesome plumber who can take on anyone, he can also control the amazing steed Yoshi. Luigi is the best sidekick anyone can get and is also an awesome plumber.