Great question,
In a nutshell, Niquests goes way beyond httpx capabilities by actually leveraging HTTP/2+ protocols, they are quite shy on its support (HTTP/2) due to the current inability of issuing concurrent requests in a synchronous context.
Moreover, this solution has a stable API, and httpx did not settle as of yet (v1 milestone).
In terms of performance, they offers similar performances in similar contexts, but keep in mind that Niquests has a higher level of features while keeping a near perfect compatibility with its origin, aka. Requests. Then, when using a multiplexed connection, you can easily beat any mainstream HTTP clients while in a synchronous context.
Finaly, no one propose DNS over HTTPS, QUIC, TLS, etc.. As well as DNSSEC, OCSP Certificate Status, Network fine tuning settings, etc..
All of those configurable with a mere simple parameter. It aim at keeping the level of simplicity you grown fund of when discovering Requests. There's more, but I am going to let you discover it by yourself. There's also an article you may find interesting called "10 reasons to quit your HTTP client" onto the README.