In my anecdotal experience, lichess has a smaller playerbase and it's most noticeable in the middle elo where certain lines or trends end up very popular in the lichess community and get overplayed. Whereas on chess.com I am more likely to see a bigger variety of openings. I had written off chess.com in favor of lichess but now that I play both I actually enjoy the chess.com user base as a differentiator between the two.