I'm in a similar situation to you in that I'm considering switching to MonoGame. However, C++ is out of the question for me because I've been working on my game for about a year in C#. Fortunately for you, your game is in the very early stages of development so switching to either shouldn't be much trouble.
If MonoGame doesn't have a content pipeline, does it have something similar? How will you import your assets with it? Furthermore, if you switch to C++, which engine will you use? MonoGame strives to be an open source XNA 4.0, so if you're more familiar with game development in XNA then chances are you're more familiar with C#, so I'd stick with that language. I have also heard that porting to MonoGame from XNA is pretty easy, but I don't have first-hand experience doing it so don't take my word for it.
Overall, I think you should look at the big picture. Which platforms would you want to sell your game on? Is it a PC game or a mobile game?
C++ is a powerful language, but it takes a while to get used to. Since it's your very first original game, I'd stick with C# since Visual Studio is a great developer tool and you'll have more problems with C++'s intricacies.