Both are beautiful in their own ways. I think ReasonML, or even SML or OCaml might be a gentler introduction since they let you write imperative code (more closely to what you probably write already) whenever you need to.

For example, I think there is a gradient from OCaml > Clojure > Haskell about using mutable data structures in your code, from Encourage > Discourage > Disallow. I think learning Haskell as your first FP might turn you off given how strict it is in this regard. So using that as a line of reasoning, using OCaml or ReasonML might be best since it lends an imperative hand whenever you need it, and lets you explore all of the amazing things FP has to offer.