Haskell has some nice things going for it. Its syntax is cleaner than OCaml's, for example. Still, it makes some things (like having a mutable variable, or a hash table) much harder than they should be. It's worthwhile looking at both over some rainy weekends. Whichever you end up preferring, you'll have learned useful things about both languages.
(OCaml is better, though ;-))
> any large cleany written codebases from which to study?
There is a big list of projects using OCaml at https://github.com/rizo/awesome-ocaml
Obviously, I can't vouch for cleanly-writtenness of any of them. Whichever you study would presumably depend on your area of expertise. There are packages there for anything from theorem proving to bioinformatics to web development. If you're into compilers, you could look at the OCaml compiler itself...