[swombat]

I doubt you can draw such a clear distinction.

I'm sure if Paul Graham had asked an equivalent of this community what he should learn to increase his market value, "Lisp" wouldn't have been the answer. At the time, probably C++ or Perl. Yet he used his knowledge of Lisp to gain a competitive advantage for his company, and it ended up making him rich. C++ and Perl may have helped him too, but that's actually less likely, since they wouldn't have given his company its competitive advantage of nimbleness and rapid changes.

You can't know in advance what will be worth learning. Except, you can: if you want to learn it, if you have a desire to understand how it works, and if that desire is storng enough to spur you to learning it, it's worth learning.

I'd add an extra indicator actually: if everyone tells you it's worth learning, chances are it's not. If everyone tells you it's a waste of time, chances are it's worth learning. Why? Because people in aggregate will point you towards popular options (duh) - and those will probably give you less of an edge. This makes the OP's question even less valid.