Nickel 1.0

You can find these in the project's README on GitHub: https://github.com/tweag/nickel#comparison

Of the two languages you mention, I've only ever worked with CUE. The things you can do with Nickel's contracts, both in terms of capabilities and ergonomics, feel pretty similar to CUE's constraints to me. Unlike CUE (but like Nix!), Nickel has functions and is a Turing complete functional programming language.