I understand that there are two main Rust client libraries for Substrate:
I've checked out the examples for both libraries and it seems to me that they are mostly the same.
From what I understand
substrate-api-client is a web3 foundation grant recipient and
subxt is the library built by Parity Technologies.
That said, I couldn't find any direct comparisons or information about the pros and cons of each library and why you would use one over the other (or maybe one is simply better than the other).