Both actually works fine. But I would personally prefer to use C++ as more people often use those and you can easily get answers for you questions about game development about those.
But , C# is easier to use compared to C++ . But in the end, it is also your personal preference too.
C# with Unity3D
Beacuse I use it and know that it is not too hard to make a simple game after you get comfortable with using Unity. At the begining you can just find tutorials of finished games and follow it, after few finished "copy" games, you will quickly be able to do something on your own.