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(Original reply:)
I'll admit it's not a factual and clear-and-cut distinction.
OCaml has warts (no multicore, although that's claimed to be very close to complete now; also no UTF-8 strings by default, and a somewhat surprising tooling, the first time at least) but I found Haskell to ask too much of me to even start -- I almost immediately grokked monads and lazy evaluation but it seemed to be too much effort for something you'd like to quickly play with. And when I saw the list of all compiler variants I almost gave up right there and then.
OCaml, as mentioned in the parentheses above, isn't without problems too but I found that I could get to a working dev stack and the ability to make a quick program and run it, much quicker compared to Haskell. Also, it's very terse.
Not the most compelling or factual of reasons but this is how I arrived at that place where I prefer OCaml for sketching ideas and not Haskell.