Nim is garbage collected / reference counted by default (but there's a way to turn it off, there's lots of GC options). It also ships with a much more batteries included standard library. Nim has operating overloading, dynamic dispatch (although I think this feature is being deprecated), various types of macros, generics, and a whole lot of other features. Nim has everything but the kitchen sink, which can be both good or bad, depending on your perspective. Nim compiles down to C code and that lets you interface with C and C++ libraries in a very native way. If someone likes programming in C but likes the syntax of Python, they'll love Nim (imo). Nim also lets you write Nim code but transpile Nim to Javascript, so it's an alternative to Typescript in some ways. Like I said, everything but the kitchen sink.

Nim's LSP is great and editor tooling is good. The testing framework is only so and so. The package manager in Nim leaves a lot to be desired. The Nim community is well established and big, but without hard data, I wouldn't say it is growing all that much. It's pretty much the same community members from 2-3 years ago that are all doing amazing work, with the addition of a few folk.

Zig is more barebones. It uses LLVM to generate machine code but a couple of backends are in the works as I understand it. It has compile time execution instead of macros, and generics are just compile time features. Zig is a lot like C in that it is simple in its feature set. For example there's no operator overloading. Which means when you read Zig, you kind of know exactly what the program is going to do. It also means code can be very verbose (especially math-y stuff). Try doing complex number arthimetic or 2-D vector calculations and the code is as verbose and ugly as C (imo). Some people will say that this code shows exactly what is going on but (again imo) it is unnecessarily verbose. If people could opt-in to operator overloading somehow it would make Zig really neat for math. I can see Zig being used for web servers, although if it segfaults because of the manual memory management it could be bad. But really the usecase for Zig is bare metal work, maybe software that needs to perform a bunch of work on data. Zig has a unique way of transforming array of structs to struct of arrays, so you get lots of speed improvements while still writing your code in an ergonomic fashion. Zig in a rather unique twist is a better C / C++ compiler than GCC or LLVM. So if you are interested in compiling a C program, you can use Zig to do that. I think Zig is a better alternative to CMake than anything else out there.

I can't speak to using testing in Zig, and I don't believe there's even a package manager at this point. There's very few libraries for doing stuff in Zig but it is growing.

I think a good way to get a sense of the community is to look for conference talks on YouTube or on HackerNews for a language. Nim has about 10 talks a year. Rust will have 30 talks roughly. Zig usually is like 5 talks, and one of them is almost always the creator of the language. Take that for what you like.

Both are great languages and I've had fun trying them out! They unfortunately don't fit my work requirements and are not personally interesting to me.