From what I see, Zen-C aims to be "C with super-powers". It still uses C pointers for arrays and strings. It transpiles to single human-readable C file without symbol mangling. No safety. Not portable (yet?).

Nim is a full, independent modern language that uses C as one of its backends. It has its own runtime, optional GC, Unicode strings, bounds checking, and a huge stdlib. You write high-level Nim code and it spits out optimized C you usually don't touch.

Here’s a little comparison I put together from what I can find in the readme and code:

    Comparison          ZenC           Nim
    
    written in          C              Self-Hosted
    targets             C              C, C++, ObjC, JS, LLVM (via nlvm), Native (in-progress)
    platforms           POSIX          Linux, Windows, MacOS, POSIX, baremetal
    mm strategy         manual/RAII    ARC, ORC(ARC with cycle collector), multiple gc, manual
    generated code      human-readable optimized
    mangling            no             yes
    
    stdlib              bare           extensive/batteries-included
    
    compile-time code   yes            yes
    macros              comptime?      AST manipulation
    
    arrays              C arrays       type and size is retained at all times
    strings             C strings      have capacity and length, support Unicode
    bounds-checking     no             yes (optional)