Neither one is "better". Every language is different, and provides different features.
C# is probably going to be easier to make a 2d side scroller, especially since there are some pretty good libraries already written that you can use for game developement (XNA, Axiom, etc). Do realize though, that C# creates .net based programs, and they will only be able run on computer that are running .net framework (windows, and some linux distros). So if you're wanting to go cross platform, forget C#.
C++ is a much more extensive language, and you'll be able to achieve more with it. It also has some really good libraries made for game developement/graphics (Direct3d, OpenGL, etc). C++ will take a lot longer to learn than C#, because it is a more robust language. Now, don't get confused when I say that you'll be able to achieve more with C++... while that is true, you might not NEED to achieve any more than you can with C#. Unless you're gonna be writing a bunch of custom shaders, and using using some advanced physics, C# should get the job done.
To make a long story short... for the job you want to get done now (2d side-scroller), C# should more than suffice for what you need. If you plan on making more complicated games in the future, maybe you want to go ahead and just learn C++... be warned though, this is a huge task to take on, and it will take you several years of dedication to get to the point where you can start making decent games.