Oooh that's a very hard question :p I'd need to sit down a bit to answer it, to be honest it could almost be its own article. They both try to accomplish the same things through very different means with very different backings (Facebook versus literally one person).

I can already tell you that ReasonML definitely has my preference these days over Elm, for the simple fact that I can keep using React with it. I love React, it's one of my favorite frameworks. If you're already familiar with it you're 50% of the way there to learn ReasonReact, whereas with Elm it really lives in its own little universe that you need to get familiar with from scratch.

I find ReasonReact's underbelly a bit messier, and the compiler is not as good nor as clear as Elm's though. And the whole OCaml layer brings some chaos too, like the multiple standard libraries available in ReasonML (OCaml's, ReasonML's, etc) which can all be very confusing when you begin.

So, I wouldn't really know what to tell you, it depends wildly of your experience, what you like, what you're looking for in the language and so on. I'd definitely recommend to give both a shot and see what you like as they're both great languages to me, even if ReasonML has my preference.