Bazel is where I started. As an ex-Google employee it's familiar, and I prefer its build language. I found Nix when looking for ways to make Python builds reproducible across machines.

Also, I get the impression that a Nix artifact is generally an entire library/package, whereas a Bazel artifact is often just one or a few files which fits well with how I prefer to use the build system. But I'm no Nix expert and that may not always be true.

If Bazel had a better external dependency story (maybe someday [0]) then I'd probably not use Nix. But currently the pair seems to work well.

[0] https://docs.google.com/document/d/1moQfNcEIttsk6vYanNKIy3Zu...

Nix doesn't have incremental builds. It's kinda all or nothing at the package level.

This explains clearly why you'd want to combine both:

https://www.tweag.io/blog/2018-03-15-bazel-nix/