Here's something I wrote out recently. The way I see it, BMX will drastically cut down your development and maintainence time/costs, which boils down to more money for you per unit work. The only disadvantage it has is no multithreading support.
C++ BMX C# DirectX10 OpenGL3 Platforms All All Windows Windows Vista All Speed of Development Slow Fast Fast - - Multithreading Yes No Yes - - Works with OpenGL Yes Yes Yes Works with DX Yes No Yes