Now that I finally have both...
I love Sulani for the relaxed atmosphere, they awesome, gorgeous islands, the diving (even if it's just a rabbit hole) the shells, the dolphins
But Mt. Komorebi has awesome paths (that you don't have to use the "hike" interaction to explore, it's just easier that way) and a beautiful environment... and a couple of skills that you just can't develop in any other world. It's got a built-in hook to keep drawing sims back time and again.
Part of the attraction of Mt. Komorebi is doubtless that it's new, and therefore I haven't had a chance to do everything yet. Because I don't rush through a pack trying to "beat" it. I weave it into my regular gameplay, and right now, that's a family of spellcasters in Glimmerbrook where the father is originally from Mt. Komorebi (because since the patch, my game has been generating a lot of sims with Japanese names, and the dad in question was a game generated spellcaster) and now that the daughter is approaching High School, they've been going back to visit every weekend.
I still plan on having them take a few weekends to Sulani too, and a few more trips to Selvadorada. Because I want the mom to get all the "explore the new world" aspirations (except city living, since that requires living in an apartment in San Myshuno) But most of their explorations will, for the moment, be at Mt. Komorebi.