I understand exactly how Luigi uses his Fireballs. This was actually a specific type of targetted buff. Basically, in standard Brawl, Luigi is already pretty good, and in most matchups, doesn't need and shouldn't get a buff. However, he loses pretty horribly to "team disjointed" (Meta Knight, Marth, Mr. Game & Watch) because the range disparity combined with his generally low mobility (Luigi Cyclone doesn't help in these matchups much) just lets them zone him way too easily so his incredible speed and power don't get to see the play they need to see for Luigi to work. Fireballs are a move Luigi can actually hit with fairly easily in those matchups, and with the cooldown reduced, Luigi should be able to rely on them more as a poke to mess with those zoning games. Getting spammy with them sometimes is effective in other matchups as a side-effect, but given how much the hit is generally lame with Fireballs, I think in the end they will still be a poke... just a much better and safer poke to throw out, a poke that you can use to disrupt things in the matchups that generally zone you to death so you can get in their face and take advantage of all the great stuff Luigi was wanting to do all along. Of course, having a functional dash attack is a nice bonus too...
Mario is much improved across the board. His general game plan shouldn't be that different, but he should be a lot more rewarding. Mario had a decent gameplan in standard Brawl, but it was hopelessly non-rewarding, largely centering around the fact that he had terrible KO options, so he was not much of a factor in the metagame. And, just like Luigi basically got a move added to his moveset (dash attack), so did Mario (dtilt). Initial impressions of Mario have been very positive, and I can say I definitely feel pretty powerful when I'm using Mario. I'm not sure I agree with certain hype elements that are putting Mario in top tier, but I'm fairly sure he's good.
Also, this has nothing to do with the project, but the fact that you main Luigi and your brother mains Mario is amusing in a silly way. I guess you guys are the Mario bros?