Java vs Python on Hadoop
Java is less dynamic than Python and more effort has been put into its VM, making it a faster language. Python is also held back by its Global Interpreter Lock, meaning it cannot push threads of a single process onto different core.
Whether this makes any significant difference depends on what you intend to do. I suspect both languages will work for you.
I am working on a project using Hadoop and it seems to natively incorporate Java and provide streaming support for Python. Is there is a significant performance impact to choosing one over the other?  I am early enough in the process where I can go either way if there is a significant performance difference one way or the other.