Nintendo is rooted in
play. Most of their games are held together by their design and the requirements to achieve a certain experience. A prime example is Mario himself, in his original design.

He only became Mario, really, because of a hat and a mustache, a decision which is rooted in the technical limitations of 8 bit sprites. Having both of these removed the need to have a mouth and hair - things that would've been harder to communicate. All together, it looked sort of like an Italian plumber. All of this began with the
game. Sonic, on the other hand, is never rooted in the experience of playing the game. Sonic's goal was always
appeal. He was designed in
a literal contest for SEGA's next mascot. There wasn't a stringent gameplay reason that he was fast - it was something to sell the Genesis/Mega Drive (Blast Processing, anyone?). Sonic/SEGA was flashy with an attitude to stand up provocatively to Nintendo.
Having this strategy against PG, safe, predictable Nintendo proved to be effective (for a time). That doesn't mean play can't have appeal, or vice versa. It's just that usually in game development, you have a main goal, and all of the aspects of the game are laser-focused towards that goal. Nintendo has been consistent in having goals for their games, and if that also happened to have marketing appeal, hey, all the better! Whereas Sonic's base
begins in his appeal, and the experience is modeled around that appeal. SEGA wasn't trying to make a
game when they made the first Sonic, they were tring to make an
icon. That's why his games include a sense of speed to this very day. Sonic = fast.

If it's not fast, it isn't Sonic.
From these two core values/principles of both of these characters/companies, you can extrapolate other values. Nintendo could be said to be more conservative with their properties, whereas SEGA is more liberal. Because Nintendo cares about retaining their brand's value (
most of the time), they don't ever wish to devalue their properties and what they stand for - for better and worse.

"Te-hee, we're so light-hearted, goofy, and weird!.....
BUT NEVER TOUCH ANY OF OUR GODDAMN PROPERTIES, YOU ABSOLUTE PEASANTS!"
Nintendo can be over-protective of their brand, to a massive fault, but that is infinitely preferable to how bad Sonic/SEGA can get. Similarly to a fault, they take the spotlight for

and against

themselves at any moment it serves them. If there is one thing Sonic is good at, it is at always leaving you with an
impression - nothing he typically does is just "forgettable", and so gimmicks sort of feel synonymous with the character now - an ever-changing name and property that serves whatever purpose it needs to at a moment's notice. If Mario/Nintendo are about towing the line,
following the rules, and being disciplined, then Sonic/SEGA would be about breaking the rules and achieving whatever they want by any means necessary. On a general level you can really notice the difference between the two simply on how

^careful Nintendo is with Mario,

^and how care
less SEGA is with Sonic.