They look similar, and it looks like their style is very similar, too. All the changes that were made are improvements, it seems. The way I see it, all the moves Lucas displayed so far work just like their Ness counterpart except with different animations and mostly improved effects.
And it's not like a character's moveset hasn't changed during the transition from one Smash to another... Ness didn't have the sparkles in 64... DK had many similar moves... and so on. Of course, you could use that argument to support Ness, but I'd really prefer it if you don't?
If I was Sakurai, and I decided that the Mother series deserves 2 reps in the first place, I wouldn't even consider having two short chubby PK-power-using kid protagonists with largely the same abilities and looks. I'd go with one of them and a villain, or one of them and a sidekick with different abilities. And Sakurai is not as unpredictable as everyone likes to claim. The man is still a game designer, not Santa Clause.