While I think both have been explored pretty thoroughly in the expanded media outside the films, Darth Vader has probably been done a bit more justice, and it sounds like he is getting a pretty major role in the upcoming Obi Wan series, so I'd love to see Maul get more attention in live action.

So as much as I love Vader, we’ve already covered the most interesting stories you could possibly tell with him in comic form to GREAT success. Maul, alternatively, has had some ok comics, but there is a big gap in his history from when he escapes from Dooku to being the leader of Crimson Dawn, imo.