The most straightforward reason to pick OCaml over F# is that OCaml's not tethered to the .NET world. Though this is also an excellent reason to pick F# over OC

The most straightforward reason to pick OCaml over F# is that OCaml's not tethered to the .NET world. Though this is also an excellent reason to pick F# over OCaml! OCaml's type and module system is generally more sophisticated than F#'s - F# doesn't have GADTs, all the functor-y goodness of OCaml's module system, or even OCaml's weird-but-brilliant structurally typed object system. The new algebraic effects that OCaml's getting imminently are pretty killer, too. F#, of course, has several awesome features that OCaml lacks.