[vlladin]

It's an interesting question, and maybe a wrong question altogether. From my current experience I've noticed that it does not really matter what you learn as long as you learn something. To go event further, choose something unrelated to your current skill set. When you learn something new, the way you perceive the world, the way you think in general changes because you have more variables to work with. You will start observing things that were always there, but the subconscious part of your brain was filtering it out because it was considered "useless". You will understand more, and at some point will be able to make connections between apparently unconnected topics, get new ideas, innovate. But if you want to learn a new skill just to monetize it (get a good payed job quickly), things are a bit different. Then you must stay in your area of expertise, and choose something close to that, something that either has potential in the near-future, of it can be monetized now. You will have to make a bit of research on that one depending on the area you live in and the number of open jobs available.