The age old question. Mario or Sonic? Which is a better 2-D Platformer?
Mario is slower, and more platform based with power-ups and two players (at least) and Sonic is faster and more based on getting to the end as fast as possible with a good score. I say Mario, just because I like the challenge of jumping through the level at a slower pace, but it's close.

What about you?

Mario for basically the same reason you had.

Meh. Sonic's only gimmick is SEGA's response to Mario's openendedness. Whereas most 2-D Marios have a few goodies hidden about levels that players are encouraged to find, 2-D Sonic games are only about collecting at least one ring and running through the stage at Mach 1 while twirling through course obstacles that resemble amusement park rides. As such, 2-D Marios have always had some sort of replay value and emphasis on exploration. 2-D Sonic games... it's almost as if SEGA wants you to not have a challenge and beat the game quickly.

Now, onto the music.

Nintendo has Koji Kondo. SEGA has contrived techno-rock music known by fans as "Sonic Rock". ...Eh... Mario themes are always made to be recognized by long time fans, invigorate new ones, and be pleasing and calming to the ears. Sonic rock is basically just bad instrumental J-pop with a few random electronica beats.(For those who thought J-pop was bad as it is...Look up the anime Fooly Cooly and the band "the pillows". They're both awesome, just so you know.)

Control scheme and other gameplay elements:

Mario's controls are very simple and arcade-y. You can walk through the level normally, just avoid obstacles. You have Fire Flowers, Mushrooms, and Invincibility Stars (and other powerups that may be game-exclusive, i.e Blue Shells, Mega Mushrooms, Ice Flowers, etc.) to help you navigate. Sonic games... "Oh, hey, let's have the player run at supersonic speed throughout loop-de-loops so the player can't see anything, then throw a random unavoidable gimmick into the course like a bottomless pit or something that'll either cause the player to attempt (and fail) to slow down and/or die!"

My vote goes towards 2-D Mario platformers/sidescrollers. I have played a lot of 2-D Sonics as well, before someone goes and bashes me, but they weren't fun to me at all and they felt very contrived and quick to 100%. Mario games have a lot of interesting features that can keep new players on the edge and old ones looking about for all sorts of cool stuff, the music is great, and the gameplay is simple.

For a Sonic and Mario fan, I'm going to say Sonic. It was 2-D gaming at it's finest! Seriously, though Mario is good. There is just something that makes Sonic 2-d games so attractive. 2-D Mario isn't the best. Heck! New Super Mario Bros. Wii wasn't that fun. But, I think Sonic 2-D wins this hands down. But, if it were ever to comes to 3-D THAT IS A DIFFERENT STORY!

I think 2D Mario is better. You can play it 100 times and still find new things, like the Warp Zone in World 1-2

Mario is better.

I'm not biased against Sega. In Sonic. You hold right on the D-Pad/Stick and the Jump Button every once in a while. It's boring.

Sonic isn't even close to challenging. Honestly, it is only fun when you play the two-player game where you play in different levels to see which player can get through the stages first. When playing the storyline, it is BORING. Just far too easy for me and the fact that you only need ONE ring to survive or pass to the next stage is stupid.

3-D Sonic games are way better than the 2-D versions, but that conversation or debate is for another thread.

Mario shall get my vote for basically all of the reasons MarioTheCellist, LiteTheIronMan, and Yoshizard mentioned.

2-D Sonic had the speed and more ways of dying. It was a more difficult game, and that's how SEGA caught people. Once you beat the game, however, you didn't want to play it anymore.

2-D Mario was a place of opportunity for hackers to make their own levels (some easy, some impossible). It was a simple and fairly easy game, but it was fun and replayable. It was the start of many hacks, speedruns, etc.

I side with the Mario 2-D games.

They both suck. Neither Mario or Sonic are good because they both have there own problems which renders them with the label I like to call
"Dragiiin ode disposal"

Oh, this one's a toughie. I grew up on both. I really like Sonic better.

In the Mario games, you get the occasional pipe shortcut. In the Sonic games, you get multiple ways to finish the level, more of an incentive to collect the collectibles (coins vs. rings) and, how Lite would say it if he were on the Sonic side, you get to destroy a usually kickass machine driven by a mad scientist that wants to enslave forest animals. 2D Mario doesn't get that much variety in the boss department. Sonic gets a new one every freakin' level.

I do have to admit though that the levels are short, but you're a hedgehog that runs faster than the speed of sound. It's supposed to be faster paced. I think that the games have the same amount of replay value, if not more in Sonic's favor because of more multiple paths

And don't forget that Sonic can be easily hacked too. I've played and seen more Sonic hacks than Mario, but then again, I'm not really looking.

Personally, I prefer Mario over Sonic.
As a young yr. 9 student, I've grown up with both Mario and Sonic. I've played Sonic more than Mario, but Mario had much more in it's 2-D games (Personally) and Sonic is Like Mario, only a different species.

Even. look at this:
http://www.youtube.com/watch?v=6D9h-4vQUHM
It's a Mario Rip off!

I still LOVE Mario AND SONIC and I think both are really good, but Mario (personally) is Better.

As a Debater, Self Preferences aren't really the best to prove a Point, but In this occasion, it seems okay.

With gameplay, I would have to say Mario. In Sonic, there are no powerups, less enemies, and no gimmicks. In Mario you are trying to avoid death. In Sonic you can just pick up a (very common) ring, get hit, and pick it up again, making you practically invincible. In the later Mario games, there are secret exits that are, well, secret. The best you can find in Sonic is the Big Ring. Mario is just addictive like that.

For music, I'm not sure. The style of both fits their respective game. In Mario, it's like the succeeded without trying. In Sonic, it's like they did it to the worst of the Genesis's ability. They're both catchy (apart from the SMB underground theme). In Sonic, each world has a new music theme, but so does Mario in the later "2.5D" games (i.e. New Super Mario Bros.) They're pretty evenly matched on this one.

Overall, I'd have to say Mario wins, not only because the "score" is 2-1, but because gameplay pretty much rules over everything else.So there you go, Mario wins.

Of course Mario, cause they have power ups, a freakin Yoshi and we get to take turns playing 2 player, in SMW and SMB3, we can get certain awesome power ups, like the Hammer bro or the Frog power up, and of course we can spare lives to eachother, and of course we can get 1-ups (on SMB3) by getting panels(the one with mushroom Fire flower and star).

I think Sonic overpowers Mario. Mario's fun, but it's slow. And basiclly all the levels are all the same, just harder and with more gimicks(Lakitus, Warp Pipes, ect.) And about power ups, I have to give it to Mario, but I don't really care. Who says you need power ups for a 2-D game to be good?! Plus Sonic's are in a TV for gosh's sake! Mario's are in a... block with a ? on it. TT....... And about boses, In Mario it's Bowser/Koopalings/Giga Bowser. In Sonic it's Eggy/G.U.N/Charecters. preety much same thing.