If time allows, I'd prefer both. If you have an arsenal of popular languages and unpopular languages (assuming you are sufficiently skilled in them), it will give you a leg up over the competition that knows fewer.

But as the other comment states, B tends to be more exciting, and allows for deeper social connections more frequently. Sure, millions of people might know java and you can say, "Hey I know java too!" But I doubt the conversation will go any further than that. However, if you have experience with a less popular language and meet (or are interviewed by) someone that also knows that language, it will create a much deeper connection.

All in all, do both if you can. If you have to choose one or the other, learn something new and become a part of a smaller community and watch it rise or fall.