It's interesting how many programmers make decisions while ignoring the wisdom of the high school girl. When in doubt, you pick something that is popular. When you are really good at it, you pick something that is going to become popular, and by choosing it, you make it more popular.
Seriously though, it really depends on where you are, market wise, and where you want to be. There are a lot jobs around here for Java programmers that understand Spring and Hibernate. However, the people hiring for those jobs are looking for competence, and little else. You won't be able to ask for a great salary in those conditions, because while good performers aren't that easy to find, the hiring pool is also pretty large.
Instead, imagine that you have 15 years of experience, and you want to remain technical. At that point, having a decade of experience on the exact same thing won't really help you. Your selling point has to be that you've seen everything, and that you are up to date with the latest and greatest. So you don't look for yet another generic job with popular tools: You have to learn shiny new things, and sell that your know-how with many tools means you'll make a lot less architectural mistakes than a youngster. At the same time, this gives you a chance of getting into a technology early, when finding experienced people is harder. You ride the top of the wave, get paid well, and can keep in the tech switching train.
Soyou need both serious knowledge of a couple of popular languages, and then to try to spend your time working on the less popular ones, that are still growing, because that's where real opportunity is.