What are your goals?

If you want to learn another language just for the fun of it (the best reason) than learn both.

Of you want to improve your tool set to be able to land a job, then there is no good answer. Probably some other high level language like Python, Java, JavaScript, C#. Etc.

Also: Zig bay be easier to get started when coming from C, because it is mostly imperative.

Rust introduces concepts from functional programming. This could be interesting for you, of you don’t have any experience in functional programming to get in touch with other programming styles. Or not, of you explicitly don’t want to learn such things.

I use both languages, and I enjoy both. Shameless plug: I’ve written a blog post ~ 2 years ago what I like about each language: https://zigurust.gitlab.io/blog/posts/three-things/