I must say up-front that this is not a strictly programming-related question and a very opinionated one. I am the lead developer of the dominant .Net CMS in my country and I don't like my own product :). Managerial decisions over what is important or not and large chunks of legacy code done before I joined gives me headache every day I go for work. Anyway, having a vast amount of experience in web industry and a very good grasp of C# and programming practices I'm designing my own CMS the past few months. Now the problem is that I'm an open source kinda guy so I have a dilemma. Should I use C# and .Net which has crippled multi-platform support or should I drop .Net entirely and start learning Java where I can create a truly open-source and cross-platform CMS? Please don't answer that I should contribute to an existing open source CMS. I ruled that out after spending a great deal of time searching for something similar in structure to what I have in mind.

I am not a native speaker, so feel free to correct my syntax or rephrase my question.