Mario seems interesting. How does Mario fare against Fox or ZSS as opposed to Luigi?
Diddy is still good, just not nearly as free. Some match-ups that he had that were even he loses now... Like Sheik or Luigi.
Both bros are pretty hard counters to fox imo. Luigi has regrab combos on him so just 1 grab is like half the stock. Mario gets lots of mileage out of grabs too and a better neutral than luigi, but struggles to get the kill unlike luigi.
Mario's just plain better against ZSS... Luigi doesn't combo her ridiculously hard, she's immune to cyclone kills, and unlike fox doesn't really put herself in positions dthrow > bair will finish her. Luigi's lack of air mobility makes him really enticing for her, too.
Overall the mario bros have similar matchups with luigi coming out on top in a few top tier MUs (pikachu, fox) but mario still has good MUs vs those characters, and even/good matchups against others like wario, yoshi, and will also contend with the likes of sheik, and falcon if they CP you somewhere precarious for sonic. (BF, dreamland, etc)
Falcon accomplishes similar stage and matchup coverage (mostly via soft counters though) but hits rosalina instead of sheik. Dabuz recently stated he believes rosalina beats sonic, so if she's your concern you might want to consider him too.
I think sheik is a more relevant threat, though I don't think sonic loses to her outright except on certain stages... This means if you lose game 1, you are in a worse position than normal vs sheik.