C# and Java have nearly the same performance profile. It may be efficient if you were to use Java for multi-OS production (and easy to use OpenGL bindings). C# can be multi-OS with Mono, but as it'd probably not hurt you any more to use Java, that's what I'd suggest.
I studied programming language performances a while back. Java uses a lot of ram, but it's CPU performance is about 5x faster than Python, and just about 2x slower than C.