MonoGame has the best future out of the open-source alternatives, though it still has its share of troubles for some with setting it up for various other platforms and building programs in them. It's also rough trying to port something more graphically intensive with many custom shaders. One big missing feature for me is hardware instancing. Still, it is the most complete in functionality, beating all of the other ones I've browsed- one in particular was full of NotImplementedExceptions!
I will be sticking to it for its multiplatform support. Before, I was entertaining the idea of doing a port such as going from XNA to pure DirectX, then to OpenGL, or sticking with C# and using the OpenTK library. MonoGame abstracts the use of OpenTK for Linux and Mac platforms.