/Scala myself, but that's a slightly different niche.
* Predictable performance; No need to worry about space leaks. * The module system is pretty nice. * Named/optional function parameters. * Being able to write filthy imperative code without needing to use monads for everything. :)
Not sure if that's true as I use neither and only did minimal functional programming in LISP. I mostly do an imperative style along lines of Cleanroom methodology. I'd like to hear veterans' opinions as to whether Ocaml's learning curve would be easier on imperative programmers than Haskell.
The most important part of these languages is really how they try to enhance your abilities to write modular programs. You might be surprised to find out languages from the 1970s and 1980s have better abstractions (e.g. functors/modules) than some designed today. :)
But fundamentally I think some of it will come down to personal choice and aesthetics at some level; e.g. I really like Haskell's syntax (principled, block structured with no semicolons) and I really like laziness in general, because it makes it really easy to 'float out' and refactor code. OCaml has a much better module system, a very fast compiler, and is very well designed and thought out IMO. None of those are dealbreakers - it's just a matter of picking your poison.
"You might be surprised to find out languages from the 1970s and 1980s have better abstractions (e.g. functors/modules) than some designed today. :)"
...is uniquely appropriate as I've been amazed by and posted so much old work on forums that there's little that surprise me. Far as abstractions, I think Ten15 (below) was most interesting I found given its potential as an integrator. Burroughs Architecture, IBM System/38, Wirth's layered design of Lilith, Genera LISP's developer flow... the best attributes of these still haven't been matched imho by modern work. Still worth remembering and factoring into one's next project if possible.