Alright, I didn't see evidence that everyone understands this well, so I'm going to explain it.
Ness' Absorption has ending lag. However, that ending lag is completely eliminated into certain actions (which you cannot punish reliably after fireball) if he actually absorbs something. This makes fireball approaching nearly out of the question, though it's still necessary, and requires GREAT patience. Remember that each fireball heals TWICE as much as the fireball would have done otherwise.
Also, Ness' Fair WALLS THE HELL OUTTA MARIO, ALMOST BETTER THAN ANY OTHER MOVE IN THE GAME.
Seriously, it's mad gay. I had to time out a Ness in a low tier tourney once, because all they did was wall with Fair and absorb. Random pokes on either side made the lead flip flop, but in the end, I outsmarted him and won.
I know it's easy to say, "But Mario doesn't get shut down by walls, he can just get inside and then just ****."
THIS IS NOT THE CASE SO MUCH WITH NESS. It's actually super duper frikken gay. MORESO THAN MARTH IMO. You can powershield Marth. Ness' Fair doesn't suffer from that.
I'm not sure who's advantage. But it's honestly so campy and down to the wire that at the time being, I wouldn't vote more than 5 points in either direction. If either player commits to an approach, the other has strong walls and options to stop it.