Last boss drops new stuff, saw a 2 piece BLM/SMN robe. It's also the hardest of the HM dungeons, apparently, so it's not your best option for farming Myth back to back. I haven't tried Copperbell yet but Haukke Manor seems pretty short, yet I'm not sure if it's speedrun-able. Pretty quick if you don't want to speed run WP, though.

You also get 90 tomes if you land on PS or either of the HM dungeons, so I'd say that's worth it.