[connorlay]

If you really want a clean slate, I think Elixir & Phoenix is the most promising stack available right now and is worth investing in. Channels, PubSub, Live View, and BEAM clustering can really simplify building interactive web applications. In my experience it has made full-stack engineering much more approachable and manageable for small development teams. The BEAM itself scales well should the need arise and Phoenix does not make many assumptions about what kind of application you are building, so you won't find yourself fighting your framework if your direction changes.

For a more conservative approach, if you are looking to rapidly scale up a team and/or desire a huge ecosystem of packages to use, then I'd say modern Ruby on Rails is the way to go for maximum initial velocity. However, in my experience Ruby on Rails requires a lot of experience and discipline to keep things "on the rails" as you grow your application, and that initial velocity can quickly vanish if you are not careful.

I say this as someone with a good amount of experience with both frameworks.