Depends on the goals of the project, what other developers are on the project, etc. In other words, what am I trying to accomplish and how does what I'm using help me get there.
For example, I might use Elixir if I was trying to onboard Rubyists to a project on the Erlang VM or Haskell if I was building out an API with Riak (because I know the Riak client in Haskell is awesome and digestive-functors is nice to work with).
As for deciding to use (ie: play with) on the side. I'll try pretty much anything (J/Julia/mocl/Erlang/agda) if a project sufficiently captures my interest regardless of community/syntax/tooling/etc.
