As abstract focus for large projects not easily summarized, I would say Nim tries to be "richly expressive" out of the box with tools to make it even moreso while Zig is more "intentionally spartan" with tools to make it expressive.
Maybe more helpfully, Zig is much closer to your "C with better syntax" than Nim. Nim is more like Ada & Lisp had a baby with better/more Python-ish syntax ("better" being subjective, as with most things).
( EDIT: But really all should make their own determinations: https://nim-lang.org )
