Both use spaced repetition. The difference is the source. Chessable sells courses authored by others: you pick a repertoire and drill it with the MoveTrainer system. Mainline skips the catalog: it looks at the games you actually play, finds where you actually go wrong, and builds the queue from there. If you don't know which course you need, Mainline answers that for you by diagnosing the weakness first. Mainline's lines are engine-verified at Stockfish depth 20+ and ordered by real master-game frequency.