If you are looking for a 5*, Atlantis is way better than an event based on the percentages. But I’d note that there are some good 4* heroes too. I for one have never pulled for guardians other than a single EHT here and there in two years (gasp!) but plan to this January. There are a few 4*'s there I’d be happy with in addition to a 5*.
So I guess it depends on what you want. Note the HOTM chance does not change so if that’s all you want, the event is better as it’s cheaper. But Atlantis has separate percentages for featured and non-featured 5* with the HOTM chance.
This doesn’t answer your question, but gives you more food for thought.
edit: I just reread your initial thread. My recommendation would be Atlantis in your situation. January has some good ones. February is decent too. Featured heroes for both include:
January: Zeline, Kunchen, Misandra, Poseidan
February: Delilah, Frida, Mok-Arr, Ariel
Personally, I like the January batch better unless you want better odds at a 5* healer and Jean.
edit2: season 3 is coming. Atlantis rotation can always change.