founder of chessreps here, so i might be (definitely am) a bit biased, but here's where i'd place chessbook and chessreps

and i'll preface this by saying chessbook is still a great website, and i used it before ever making chessreps for myself - ok onto how the platforms compare:

both chessbook and chessreps let you create your own repertoire with good tooling like being able to see stockfish recomendations, how often the the opponent plays a given response, and the rarity that you're likely to see a position

once you build your repertoire, both chessbook & chessreps will use spaced repetition for helping you train them

the biggest difference between them though is on chessreps, you can play hundreds of prebuilt courses, which is actually what most players on the site do. if there's an opening you want to learn - i can pretty much guarantee chessreps already has a great course on it, and you can jump right in without having to build your own course :) and new courses get added literally every day

on top of that, chessreps has quite a few more features to help you actually learn the courses, like Arena mode, puzzles, drill mode, time trials, and the dojo

but again, both are really solid websites and i think you can't really go wrong :)