I'm obviously biased since I'm one of the Rebar3 maintainers but: I prefer a declarative approach to config files and whatnot; composable profiles are really neat; the plugin system for reusable tasks; dialyzer "just works" with it; the ability to compile and build projects in non-elixir BEAM languages.
That being said, Mix does have its advantages in other ways (easier to extend for one-off scripts, for example, since it won't actually need a whole plugin; the ability for mixed-language projects with Elixir and Erlang), and they're clearly the reason (with Hex) why we have good package management today.
Again, I'm not ready to say one language has better tooling than the other; it's just that I feel it's a far cry from the situation from a few years ago where Elixir was just miles ahead -- the gap has closed since then.
That being said, Mix does have its advantages in other ways (easier to extend for one-off scripts, for example, since it won't actually need a whole plugin; the ability for mixed-language projects with Elixir and Erlang), and they're clearly the reason (with Hex) why we have good package management today.
Again, I'm not ready to say one language has better tooling than the other; it's just that I feel it's a far cry from the situation from a few years ago where Elixir was just miles ahead -- the gap has closed since then.