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From my experiences with it, in LibGen you have to search exactly what you want. A simple typo ruins it. Z-library is a better in that regards, so typos don't really ruin the search, allowing you to find stuff more easily.

I don't know whether they have a different dataset of books, but the search engine is definitely vastly superior. Not only in looks, but most importantly in terms of results.

I think Z-lib indexes libgen's recent uploads much faster, so it makes a difference for newer books.