LLVM-*-Windows-X64.tar.xz is created by a github action (.github/workflows/release-binaries.yml), which I think is new for the 19 release.
clang+llvm-*-x86_64-pc-windows-msvc.tar.xz is created by llvm/utils/release/build_llvm_release.bat which is what we’ve been using to build the official Windows release binaries so far. That also builds the installers (LLVM-19.1.1-win{32,64}.exe).
The situation is a bit confusing for everyone.
One concrete benefit of using the “original binaries” is that they’re built with PGO, rpmalloc, etc. so the toolchain is much faster. That script is also a bit stricter about ensuring that (most) tests pass before creating the binaries.
We reduced the number of included targets due to limitations of the NSIS installer framework. I thought people normally used Emscripten for Wasm, but if it’s widely useful we can add it back.