I don't know, in general, that they're the "most useful" if utility is defined as conferring the ability to write better programs more quickly, more understandably, &c. But they are "most useful" in the sense that they confer such benefits with a minimum learning curve and with little deviation from the already understood mainstream programming paradigm. For example, monads are an incredibly useful feature, but the (perceived) barriers to understanding are high enough that no mainstream language will feature them as centrally as Haskell does.

I don't mean this as a sleight against anyone, at all—merely as a description. There are a lot of useful things which are outside the purview of the mainstream, and there's no reason to expect that nonetheless useful features of fp will necessarily make it into the mainstream. (And there are certainly cases where functional programming as a whole, and not just cribbed features, is desirable and advantageous, as in Jane Street's case, so I do hope that more non-academics come to see it as a valid paradigm for certain tasks.)

This write-up describes the basics of ocaml and haskell and some of their individual strengths. (along with scala and scheme)

Extensions to the type system are really different, OCaml's is more flexible and specific, but Haskell's is very convenient, but by the time that becomes an issue you'll probably know a lot more of how the languages differ.

Ocaml: strict and impure if needed, but has everything you need to write great functional code (and not much more, which makes Ocaml actually fun to read; I can't say the same of that horror they call C++). I tend to think of it as a functional C. It's a bare-bones, simple language but it's awesome.

Haskell uses type classes for what Ocaml achieves more generally (but also with more work) with functors. Type classes are prettier. Functors handle complex cases (multiple related types) better. Hard call which is "better".

Haskell is more mind-blowing, Ocaml is more practical for most purposes, but probably not as practical as Scala. I like them both (and Scala).

Sadly, I wouldn't bet on either reaching the mainstream. Haskell is lazy, which makes it really hard to reason about memory usage. Ocaml's a great language, but the libraries available are quite poor and the effort to get it ready for multicore is DOA. I wish these languages have more of a future than I think they actually do. Now we're in the "raiding phase" where languages like Scala are borrowing awesomeness from them just like Ruby and Python did to Lisp 15 years ago.

On that, I think Scala's actually a good bet for the next 5 years, and not a bad language at all, although it's hard to learn because most of the online resources are pretty poor. (Odersky's Scala book is great.)