By all accounts, both OCaml and Haskell have sufficiently performant compilers and runtimes for almost anything. Picking between them on the basis of pure performance seems silly to me. You've come this far -- moving away from the obviously most low-level and performant languages (C, C++, etc.) in the name of clearer, more succinct, more expressive, higher-level code. So why, when the performance differences involved are much smaller, switch to that criteria now?
I'd go with some broader criteria -- if you want pervasive parallelism, then Haskell's the better choice. If you want genuinely pervasive mutation, then OCaml is better.
If you want only very coarse parallelism at best, and you intend to stick with mostly functional structures, then pick based on something else, like syntax (I think Haskell is much nicer here, but that's subjective) or available libraries (Haskell wins on quantity/availability, but OCaml might edge it out in the graphics department nonetheless).
I don't think you'll go wrong either way