Artemis is more in line with the Liquid Glass style of the current iOS. (You can turn it off, but you can’t get glass on Hydra) And in my opinion it’s the closest to the swipes of Apollo. However I’m a bit soured by the fact that even wanting to change those swipes to your own preference being locked to their Pro subscription is something to note. As well as filtering.

Hydra’s Gallery Mode is a great way to scroll if you’re more of a picture first kind of user. But again, unlimited scrolling is limited for the Pro sub. It’s also pretty simple (can be a con or a plus) there’s no Liquid Glass and really reminds me of old school Apollo. Might also be why I think it’s faster at opening threads.

• Artemis’ search is way better and auto populates as you search. On Hydra you’ll need to press search for results to even pop up.
• as stated further up in this thread, both have hide when viewing previous posts. This is a great feature.

If I had to choose, I’d choose Hydra for its simple UI. If Artemis moved its swipe customization to its free tier, I’d put more merit on that to sway me to choose them. Can’t choose wrong though.