I would assume that these two use cases are basically completely separate.

Auto-translate from C to Rust would serve as a great step to starting a porting project. Now you can incrementally re-write the "basically C" auto-ported code to "proper Rust" without dealing with FFI and other pains that come from function-by-function ports.

Fil-C is great for running software that you don't want to port. (Or don't yet have the resources to port.)

Interestingly there is probably a gap between the two. When your project is pure C you can use Fil-C. However I don't think Fil-C supports Rust. So assuming that the initial C to Rust translation doesn't produce 100% safe code (I'm not aware of any current tools that do this) you have this middle state where you can no longer compile with Fil-C but have lots of unsafe Rust code. So maybe there is a use case for Fil-Rust where you compile your Rust program so that even unsafe blocks are in fact safe. This could be used until you complete the port.