Can't speak for OCaml yet; I've wanted to learn for a while but decided to go with Haskell first.
On the Haskell side of things--it's biggest seller (functional purity) is also the biggest downside. To give you an idea, papers have been written about the best way to implement common data structures purely functionally.
From what I've seen, OCaml has less "abstraction overhead" because it can dip into imperative code. While Haskell is really fun, it definitely has moments where you want to beat your head against your desk.