IMO PyO3 project is more useful. C++ and Rust cover similar field, while Python and Rust are good at so vastly different things that they complement each other much more.
The purpose of C++/Rust bridging is to allow adding Rust code to existing C++ projects. PyO3 lets you embed a Python interpreter in an otherwise Rust-only application. They serve two different markets - though, if you really needed to, you could embed both C++ libraries and CPython in the same Rust app.
