Mario, although my preference for Mario nowadays has been almost exclusive in form of ROM hacks, because the new 2D games are too sterile and basic for my taste. Mario Maker has potential, but that's not a standalone game, even if it has Nintendo made levels.
I know there's no reason for Nintendo to make a "core" platformer, they're focusing at being accessible for beginners, and that's super ok.
But I wish there were some optional stuff with hard and creative challenges for experienced players.
*Core platformer means a game that focuses on platforming as its main thing, it's not about exploring and finding collectibles or utilizing gimmicks that completely change the gameplay, or any game where the platforming is too much of a basic thing or a secondary aspect to the game.
Sure, Mario has always been pretty simple and beginner friendly for its time, but... I just feel like at least in the 2D games (which again, is what I prefer for platformers), it has been significantly less interesting. Not in terms of difficulty, but there's a general lack of surprising ideas in level design and challenges (again, not the level of difficulty, but the obstacles and the way the levels are arranged).
It doesn't help that the presentation for the 2D games hasn't left the NSMB style, which is from 2006 or so. The older 2D games changed styles and setting more frequently, so it made things at least feel fresh, even if there weren't many crazy differences in every game.
I know Nintendo has it in them to make interesting stuff, Mario Maker 2 proves it, there are some cool official levels and a bunch of nice mechanics, but... they're not in a standalone game, and sadly, if a new 2D game comes out it will be like NSMB again, most likely.
Now... for Sonic.
I played a lot of old Sonic games, almost all of the 90s games at least, including all the spinoffs and whatnot. I enjoy my memories of those games, and I can enjoy them, even with their flaws. But I'm not a big Sonic fan per say, I haven't played many of the modern games, only the Advance games and probably something else I forgot.
I also don't care about the gameplay of 3D Sonic, at least not by looking at it. Sonic Adventure 1 looks like it would be fun for me, and I have yet to finish watching raocow play Adventure 2 (which has cool looking moments, but I don't have much to say about it).
What I do like about Sonic is actually the fangame factor. The fans are free to do whatever they want with fangames, and that by itself makes me excited because it creates a community where you're free to experiment and there are indeed a lot of fangames I think are amazing.
I gotta say I appreciate Sonic music a lot more than Mario music though. I don't care much about the buttrock Sonic songs, but they grew on me by watching raocow play them, but I absolutely love the 90s Sonic OSTs.
Mario songs are cool and all, but at least in the main series, there aren't a good variety of songs, while in Sonic, each new location has its own song. Mario spinoffs have significantly better music, especially the RPGs and even the sports games and Mario Kart have a lot of cool songs. So I don't know why the main games tend to keep a standard of 1 song per level type (desert, grass, mountains, water, etc).
In terms of visuals, Sonic also usually wins for me. Especially the older games with the more surreal and super colorful aesthetics mixed with the ecofuturistic vibes it has going on. I love that stuff.
While for Mario, my favorite games visually are the RPGs and the Mario Land games for having a lot of variety in locations (especially the second game), and in those games the worlds feel so alive.
One thing I also appreciate about Sonic is that they at least tried to put the other characters in more games to be playable. While for Mario, they're usually just in spinoffs, unless they're Mario, Luigi Toad, Peach or Rosalina. And they also usually don't have many differences in gameplay, which Sonic characters at least try to differentiate a little more, even if they have a lot of the same core abilities.
I'm gonna end this comment here, because it's getting too long.
Bacon is kind of a pig vegetable.