Cartographer and Navigator both look at the next five cards and let you discard them. Cartographer lets you discard only a subset though. Aside from that, they each have a vanilla bonus tacked on -- Cartographer is a cantrip and Navigator is terminal Silver.
Cartographer only looks at 4 cards, unless you're also counting the card that is drawn.
I do think they fill different niches though. Navigator is pretty much just for cycling so you can play your best cards often. Cartographer, on the other hand, is for filtering and pairing up combo cards. I don't really see Cartographer as an improvement over Navigator. I mean, Navigator filled its role fine and Cartographer fills a different role.
Thief vs. NB, on the other hand, both fulfill the same role of stealing treasure. NB simply does it better (usually). Likewise, Scavenger and Chancellor are both deck flippers.