Well if mario had a star or metal-cap hed probably win, otherwise he can still punch/kick/sit on sonic, sonic's only moves involve his spikes or w/e sure hes fast but not eye-blindingly fast and if I'm not mistaken he requires momentum to really get going anyway, i know some people will say sonic could go super, but its been proven in the past that metal mario or a star could deal with otherwise invincible enemies.
Depends on what Mario/Sonic we are dealing with here and which games we are using abilties from.If we assume all abilties from every game base Sonic has an advantage with heat-seeking jump attacks from Sonic Adventuer games and upwards but Mario could counter with certain ones of his power-ups.
Hammer Bros Mario-Could nail Sonic as he attacks with Uber-owerful hammers that even wasted invincble enemies from his games
Tanuki Mario-Flight and the ability to dodge any of his attacks by turning to stone.He could also outlast Super Sonic by staying a statue for his entire time limit as SS
Star Mario-Basically as inincble like SS but temporary[probably too much so to make a difference]
Any other Flying Mario-Can outmanuver Sonic
Its also a question of Stamina.Regular game Mario can take 2-3 hits before dying normally but Sonic lives so long as he has at least a couple rings stored up.Advantage Sonic even assuming a regular amount of Mario-power-ups/Rings from respective games were available
Only way i see Mario winning is 1] Frog Mario takes the battle underwater where he has multi-game experience and Sonic doesnt or 2] Allow Mario his Mario RPG hit Points stats items and weapons where the advantage goes to him a little to much
Round One: Mario Goes To Jump On Sonic, Lands On Spikes.
Round Two: Mario Fires Off 2 Fireballs, Killing Sonic.
Round Three: Tanuki (I Call It Tootankoopi For Some Reason) Can Turn To Stone As He Jumps Above Sonic, Crushing The Spikes And Killing Sonic.
The Battle Can Go Both Ways. Mine Just Votes For Mario 馃槃
I don't think Mario turning to stone will be too much of a problem for Sonic. Lest we forget the greater majority of enimies he fights all robots, some of which are stupidly big.
My view on this, Mario takes the fight only as long as he can keep Sonic away from him with projectiles, up close Sonic's dash attacks'll gut the plumber.