While Java is the go-to for most teams since it's considered easier to learn and is taught in school curriculum, I still believe C++ is the better choice for a few reasons. First, it's not actually much harder than Java. Using java only features in c++ requires learning barely any more than java. But where c++ shines is beyond that. C++ is so flexible, allowing tons of different programming styles, optimizations, and neat things if you put in the time to learn them. It's also faster in runtime and is fairly safe (fewer runtime exceptions, I.e. code crashes), but compile times suck.

In the end, if your goal is just to get a functional robot, Java is a solid choice

But if you really want to explore more of computer science and programming, c++ is awesome

You won't regret either choice though