enrichr() performs a hypergeometric test comparing the set of "significant" genes against the "universe" (or background) genes. GSEA() is a Komolgorov-Smirnov test on the whole ranked gene list, testing if some category (e.g., a KEGG pathway) is more abundant at the top of the list than expected by chance. GSEA is generally considered preferable, as 1) it is considered more sensitive in general, as it can detect small, but concerted expression changes, and 2) it doesn't need the somewhat subjective step of defining which are the "significant" genes.