Both projects seem to be alive and well
From my point of view, the main difference is that they have different codebases and code architectures.
First of, they both implement the Solid specification. So any app that works with Solid should work with them. I guess they have some minor differences for stuff that’s not explicit in the protocol. For example, iirc, CSS sends a different status code when putting/updating a resource than NSS does. But in a normal case, this shouldn’t impact the app functionality (as both send a 2xx status on success).
So the difference is more about the inside workings, than the outside functionality. CSS is modular, so you can easily replace some components. A good example is, that you can switch between a file-based storage or a memory-based storage by only replacing some components (or in this case just changing a config, as both components are already present in the codebase).
I guess there is more to say about it, but those are the main points I know of 
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