Depends on what you want to do in the future. Python is good in general because you can use it for all kinds of things, it's used at lots of startups, and it's a quick jump from Python to Ruby or JS. Java is used at a lot of big companies and some startups, but is kind of out of fashion at the moment. Java code can be a lot faster than Python code, though.
If you really master C++, you'll have a really easy time learning Java and a pretty easy time learning Python.