Alright, I'll state the things that'll probably happen in the Death Battle:
This is judging from things like Bowser vs Ganon and Metal Sonic vs Zero. Also, keep in mind that they don't really use our ratings and are more likely to go by OBD if anything.
AP
That will probably go to Mario. In terms of feats, I doubt they're going to include the higher stuff, like the various pocket dimension creation feats of both verses, the statements, etc. Beating people powered by the biosphere or by a Grand Star will probably not be counted. So that leaves the feats of each other, in which Mario has a direct city level feat, compared to Sonic, who doesn't. If they do scale, which may happen, then it gets a whole lot more complicated. Super Sonic will probably be stronger than Mario, but Sonic won't scale to tier 2, let alone infinite speed, and the time limit will come into play. Paper Mario won't be used at all.
Yeah, just throw Solaris, Dreamy Bowser, Egg Wizard/Salamander, Antasma, etc. out the window. Their feats aren't getting used.
Speed
That's surprisingly closer than I thought originally. At first, I was going to say that Sonic was a shoe-in, as there's no way Mario's MFTL+ reactions will come into play. But then I thought of how they did Metal Sonic and said he was like, Mach 36 or something, which was <<<<< the lightning speed Quick Man. Mario has feats of dodging lightning. However, Sonic has the lightspeed dash and the Guinness statement to rely on (statements will mean much more for speed than AP), and again, Super Sonic will undoubtedly be quicker. However, time limit.
Durability
I have literally no idea what their best durability feats are. They'd probably use scaling here though, like they did in Ryu vs Jin. Whatever they said for Shadow, they'll say for Sonic, and they'll scale Mario's durability to taking hits from Bowser.
Items
Yeah, they don't cancel out, and I'm pretty sure Death Battle knows it. Mario's got the clear advantage in this category in both, versatility and usefulness. Gold Flower, Ice Flower, Boo Mushroom, the aforementioned White Tanooki, Vanish Cap, various hammers, Cappy, and more. Sonic has the Indigo Asteroid and Violet Void that's really useful, and they only last a few seconds. But of course, how could I forget the Chaos Emeralds? Like seriously, how? I brought up Super Sonic like, three times already? They're the game changer and the they'll get their own section.
Skills
Smash isn't going to be used. That's without a doubt. And Sonic is a better h2h fighter than Mario with much better h2h abilities going by feats. Mario's a brawler. Sonic's more than that. Mario has Firebrand to close the gap but Sonic has his wind abilities, Time Break, and more. Without any items, Sonic holds the definite skill advantage. It's less so with items taken into consideration (like Boomerang Mario or Hammer Mario or hammers in general), but still Sonic...maybe.
Experience
Mario's had more games, and therefore more adventures, Mario's older, and Mario's been adventuring since infancy. Mario takes that pretty handily.
Transformations
Yeah, Sonic's not getting Hyper, Excalibur, or Darkspine for obvious reasons. See items for how I see that ending up.
Like I said, Super Sonic is getting its own section, so no mention of it here.
Hax
Sonic has time manipulation in base, but he doesn't necessarily have time stop, and Time Break will have that weakness of slowing Sonic down as well. As stated above, Indigo Asteroid and Violet Void are large boons, but they're temporary. Mario has Gold Flower, Double Cherries, and Cappy. Potentially Ice Flower, given what we've seen ice do in Death Battle (Jak and Daxter vs Rachet and Clank & Glacius vs SubZero). However, they can be hit out of Mario. So it's temporariness against feebleness.
Invulnerability
This counts as the section...
Mario and Sonic both have ways to be invincible, and given how Death Battle likes to use NLFs, they'll be completely invincible (lookin' at you, Jotaro).
So this is pretty much Super Sonic vs Star Mario, Metal Mario, Mega Mario, Vanish Mario, and Statue Mario. Mario has more, but Super Sonic is faaaaar better than either of those. But most importantly, it's Super Sonic vs White Tanooki Mario, and neither can hurt the other. Difference is, Super Forms have the time limit according to Death Battle. White Tanooki doesn't. Plus, they brought up in Mewtwo vs Shadow that while the body is invulnerable, other parts aren't. Mario has ways to attack that which is not the body. Sonic doesn't. Sonic does have one huge advantage though, and that's that once he leaves his Super Form, he returns to peak condition.
Conclusion
Given Death Battle, this'll be close. While the above does favor Mario, due to initial AP advantage, items that can put Sonic down quickly, and sheer versatility, it's not that easy. IFF Mario runs out of items like last time, Sonic will beat him due to the skill difference and probable speed advantage. Thing is, I don't see it lasting that long. Mario's advantages with his power ups last much longer than Sonic's, with quite a few of them not having the time limit that'll take up well...time. Sonic's fast, but given how they did Shadow, Knuckles, and Metal Sonic, he won't be fast enough to outlast Mario or do a hit-an-run tactic, and Super Sonic will struggle with Mario's invincible forms.
This will wind up like a gruesomer version of the One Minute Melee.