

I wouldn’t bother pitting then against each other. They’re very different, but if you like Firefly, you should check out The Expanse.
Personally, though, I recommend the books. I watched the first few episodes of the show first and was underwhelmed, but then checked out the books and was hooked right away.

They’re also different enough that people will disagree on taste.
The worldbuilding of the expanse is head and shoulders better than Firefly. But Firefly’s strength was characters: the writing and acting made the crew so loveable. I like the crew of the Roc, but I never felt the same level of connection to them.