Mario has insane agility, some of the best in the game. In the air, he moves very quickly and his aerials have low ending lag and landing lag and **** like that. If you watch guys like Boss play you will see an extremely fast character on display. Ness has his tricks but he's gimped more easily than Mario IMO and Mario's lack of kill moves is kinda evened out by his ability to rob chars of their stock at 20%. Unless he's facing ROB or something, that's a nice advantage. Cape is ****. Fireball is wayyy more spammable than anything Ness has. FLUDD is unique and the potential is untapped(ill pump..), that and Ness' kill moves aren't that great to begin with. Easily better than Mario's, but they are all completely predictable except for bthrow because it comes out of a grab.
Mario can also camp a lot better than people give him credit for. Fireball is oh so spammable and cape blocks all projectiles. The projectile returning effect lasts so long after the actual swing that if you had Fox spamming laser against Mario spamming cape, Fox would be damaged twice as fast as Mario. If you had Falco doing it, Mario would probably not get hit. Mario also has his tricks. FLUDD can be spammed like a mother****er against Snake to just piss him off beyond belief. It's an extremely useful tool when used correctly. I've actually used FIHL to get a half charged F smash on my opponent before he regained control.
But they both have their problems. Ness has to select his moves very carefully and his boxing game isn't too great. Mario's boxing game is just fine but he suffers from one problem, he's got no weapon. If you look at the high tiers, they either have some sort of sword or projectile or insane range that allows them to **** those without a weapon or anything like that(like falcon..). Wario is an exception, but he has his air handling.
Anyways, Mario and Ness..it's hard to say which is really better but you should think before dogging Mario. They both have stupid game flaws that screw them, Mario being infinite'd by D3 and Ness having that grab release BS.