While I can argue in favor of tooling being roughly on-par (sometimes better, sometimes worse), I can't really discuss syntax since this is often just related to taste more than anything.
My stronger argument would usually be in favor of the community you end up with. While both communities are starting to merge and grow closer over time, your average Erlang dev is likely to work on infrastructure components, and your average Elixir dev is likely to be working on web apps. I personally like working on the former better, so there's more limited interest for me in terms of most Elixir jobs anyway, even if it's not like no infra work exists in Elixir.
Both can have a great time working on APIs of any kind, and both can reuse most libraries of each other's community, so as I said, this is starting to become a moot point though. I personally have no problem with either.
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.