I admit: I am entirely biased in favor of Mario. So I'm going to step away and look at them comparatively.
In Mario's case, it's been an ongoing series with little change in the gameplay. Your goal is still to rescue, collect, and complete levels. You get bonuses for going quicker, you get power ups that help you proceed through the level, and (in most cases) there is a gradual challenge curve that makes it so you're not doing levels that are way too easy if you're a more advanced player. In newer Mario games, they made it more casual-gamer friendly by allowing people to get a suit if you die too many times. This suit will make you invincible and give you a power. It's age appropriate so anyone can play it. It's an easy pick-up-and-play game series overall, and it branches to every Nintendo console. Some of the Mario games, however, have very poor character selection and can cause people to argue over things. (Mario Kart and Mario Party are what my boyfriend and I call the friendship-killers because of how competitive they can be.) Another thing is that there isn't enough variety between the games. Each one can stand on its own, but they are all similar to where it's easy to confuse levels with other games in the series unless you know the game well.
On to Sonic. I admit, I have only watched people play Sonic, so I don't know how the controls are, but it looks like it's fairly simple in terms of gameplay. Each game has a different and interesting story. The originals were hyped up for good reason: the characters were interesting, the graphics were great for the time period (and are still pretty good), the story is pretty easy to follow overall and the levels are various and memorable. On the negative side, some of the Sonic games were not well done. In Sonic 2006, there are a lot of places where there were glitches, there were gameplay issues, there were storyline issues that eventually led to Sonic 2006 being eliminated from the timeline, the levels had no difficulty curve and were HARD. Not challenging, but just outright difficult. In Sonic Heroes, one of the Sonic games that got a very bad reputation, it had a very good idea by having different characters in different teams and you could play with the teams. However, the execution wasn't what the fans were looking for. In the Sonic games for Wii, they were all strayed so far from the original Sonic that they could've been in a different series entirely (with the exception of Sonic Colors). With Sonic games, I've found that instead of having a decent game every time, it can tend to be a hit-or-miss.
So in the end....I'd say Mario over Sonic. Even if my boyfriend disagrees.
I admit: I am entirely biased in favor of Mario. So I'm going to step away and look at them comparatively.
In Mario's case, it's been an ongoing series with little change in the gameplay. Your goal is still to rescue, collect, and complete levels. You get bonuses for going quicker, you get power ups that help you proceed through the level, and (in most cases) there is a gradual challenge curve that makes it so you're not doing levels that are way too easy if you're a more advanced player. In newer Mario games, they made it more casual-gamer friendly by allowing people to get a suit if you die too many times. This suit will make you invincible and give you a power. It's age appropriate so anyone can play it. It's an easy pick-up-and-play game series overall, and it branches to every Nintendo console. Some of the Mario games, however, have very poor character selection and can cause people to argue over things. (Mario Kart and Mario Party are what my boyfriend and I call the friendship-killers because of how competitive they can be.) Another thing is that there isn't enough variety between the games. Each one can stand on its own, but they are all similar to where it's easy to confuse levels with other games in the series unless you know the game well.
On to Sonic. I admit, I have only watched people play Sonic, so I don't know how the controls are, but it looks like it's fairly simple in terms of gameplay. Each game has a different and interesting story. The originals were hyped up for good reason: the characters were interesting, the graphics were great for the time period (and are still pretty good), the story is pretty easy to follow overall and the levels are various and memorable. On the negative side, some of the Sonic games were not well done. In Sonic 2006, there are a lot of places where there were glitches, there were gameplay issues, there were storyline issues that eventually led to Sonic 2006 being eliminated from the timeline, the levels had no difficulty curve and were HARD. Not challenging, but just outright difficult. In Sonic Heroes, one of the Sonic games that got a very bad reputation, it had a very good idea by having different characters in different teams and you could play with the teams. However, the execution wasn't what the fans were looking for. In the Sonic games for Wii, they were all strayed so far from the original Sonic that they could've been in a different series entirely (with the exception of Sonic Colors). With Sonic games, I've found that instead of having a decent game every time, it can tend to be a hit-or-miss.
So in the end....I'd say Mario over Sonic. Even if my boyfriend disagrees.