Well, most important is probably availability and stability of domain relevant libraries in either language. There are areas Haskell has well covered and areas where it's thinner...
It's also been the case historically that Haskell had much better concurrency support - I'm not sure whether that's still true - in which case a more parallel workload might push Haskell-wards (if it's not so parallel that simply spinning up N entirely separate processes make sense, in which case concurrency support doesn't matter).
I'm not familiar enough with the current state of OCaml, or with Haskell libraries outside the domains I've been playing in, to really give a thorough run-down of specific domains, sadly. I'd be interested to see it if someone else took a swing.
