[aggieben]

I usually decide based on some combination of a) do I know the language/platform well b) is this for a client or for fun c) is this for learning, and d) is this a mainstream platform, then (e) if not, then does it have an advantage solving an important problem at hand?

If (b) for a client, then (a)/(d)/(e) are important. I know the .NET platform best and it's a mainstream platform, so I tend toward that one. If your strongest platform is X, then I think usually you should use X in general, unless something else has a distinct advantage (i.e., write device drivers in C or C++, not C#; use Y because it has libraries available that would save you tons of work, etc).

If (b) for fun, then I pick whatever I feel like piddling around with. Sometimes Node, Go, F#, C++, whatever.

If (c) for learning, then I pick something useful to me or something I need to get better at. Right now this is F# and C++ for me, but will sometimes be something esoteric.