I've used both in production and I must say I'm a very big proponent of both. Actor model of concurrent computation I think maps very well to, at least my, way of thinking and I think is the most productive way of writing distributed systems.

Elixir/Erlang/OTP:
+ Very mature, very well thought out. While the newer stuff may feel a still under construction (string handling, date handling) all the concurrency primitives are rock solid, and by rock I mean diamond.
+ Elixir is simply a great language and you can get quickly very productive in it once you grasp the actor/process model of BEAM
+ Has one very big advantage over akka that actors can receive messages in different order than they were sent. That of course can cause some headaches if not handled carefully, but 99% of the time straight up leads to nicer and simpler programs. Really a lot of akka code very often is just written to deal with the order in which the messages may arrive.
+ Truly resilient with very good error recovery design once you know how to work with it. I still don't know more graceful and productive way of recovering from failures in a running system.

- For doing any expensive computations it's slow and it's a fact. Not much can be done about it.
- Library coverage is 7/10 or 8/10, but these few missing points can sometimes make a big difference.

Scala/Akka:
+/- I love Scala language and static typing, but one must be honest that it's much more complex. You can learn Elixir (without macros) in an afternoon. One really needs to take some time and think it through to utilize Scala properly and a true mastery lies even further. To be fair proficiency with Elixir macros also requires a considerable effort, but one can go very far with Elixir without writing macros, while with Scala already the upfront cost is pretty high.
+ Up to my best knowledge Akka Streams is completely unique and completely amazing library that is gaining support throughout library ecosystem. This is one point where Scala/Akka completely outshines everything else. Streams are such a great abstraction and gave a huge productivity boost to most of the projects I was working on. Compared to that, elixir GenStage feels much less robust and polished.
+ Speed of JVM should be enough for 99% of applications, and with that respect it wins with BEAM.
+ Java/Scala library ecosystem is very deep and is simply much more comprehensive than Elixir/Erlang one.

- There are places around akka that still feel a bit immature/adhoc, but the library is steadily improving. It just is not as mature as BEAM/OTP.
- Over small and mid size projects I think I was more productive with Elixir. Meaning, if I had the same amount of time, I could implement more features in less time using Elixir. But it could be just a personal thing.

Overall I think both are fantastic platforms and I'm happy to have both of them to choose for each project. If I were to chose what to select today:

- Choose Elixir/OTP for a system where we need to do a lot of io but not much computation and we're sure existing libraries cover our needs. Very big plus if we need it to be resilient.
- Choose Scala/Akka if we need speed or call any existing JVM libraries. Very big plus if your project could use Akka Streams.