If you don't have a preference think about a project that you want to do and go from there . The best way to learn is by doing . Pick task that you clearly understand the business requirements for like an online address book. Think about "What would be the technical requirements for that ? ". Having a clear objective makes it easier to learn.
It really depends on what you enjoy doing most. Do you enjoy back-end server development or batch jobs? Then I would recommend getting back into Java. There are a lot of great books out there and freely available websites. The Spring framework seems to be extremely popular so I recommend learning about it.
I highly recommend using IntelliJ IDEA if you do go back into Java. It has great automatic suggestions for how to update your Java code to more recent standards.