It's tough to tell, each technology has it's posetives and negatives.
Try a Google search "Compare .Net and Java"
Or simply go with what you are most familiar with. It is easier to learn and grow with what you are familiar with than to switch to a completely new language.
I don't think .Net is open source, it is proprietary and how it changes is completely in the control of a company.
Java is open source , and is shaped by the community rather than one company.
I've had it with vendor/proprietary technologies because, the changes they make to the language are totally unpredictable.
It's really up to you which direciton you want to go.