Elixir releases are a subset of distillery that’s correct. Currently there’s a bunch of overlap in functionality, as distillery basically had/has to do all the stuff elixir releases do. Especially as it’s still used for elixir <1.9 projects. Distillery also does more than elixir core releases. Most notably is the support for hot code updates in distillery.
As for when to use which: If you’re on elixir 1.9 (possibly without distillery already) I’d use core releases unless you need features only distillery provides. I’ve also heard people in slack note that elixir releases seem smaller than distillery ones, but I cannot confirm that, but might be worth looking into if file size is a concern.
I’d not try to mix them, as they are similar, but also very different: e.g. the cli in bin/ is completely different. Instead of changing distillery though I’d rather hope that distillery’s functionality will over time be extracted in (possibly smaller, more purpose build) new libraries, which hook into core releases using the :steps configuration available. I’m not sure how well this is possible for e.g. hot updates, but I’d expect adding things like build hooks or custom cli commands should be possible this way.
