Personally I like Haskell. As for reasons to choose it over Haskell, I find it to be a bit simpler and easier to get things done. Mutable state takes less work. For places where you might drop down to C for bit twiddly speed it is easier to replace C with OCaml than Haskell. You can still do lazy in OCaml though you have to choose it. It is a part of the standard library. Being an eager language it's evaluation model is easier to understand.

OCaml vs Scheme is harder for me to answer I haven't used Scheme that much. Static typing if you like that sort of thing.