Not sure if it makes sense, just trying to explain what's going on in my head. As i have debated that exact question myself, internally.
Culture and rules: In this case this means how nudity is handled, whether anyone will care if you post spiders or gore without warning, some specific rules about AI-generated art, etc.
I can confirm that mastodon.art feels cozy and well-moderated, been there for years, also a financial supporter. Another sign that they care is that they've closed registrations, instead of trying to instantly moderate 3x the users. They've recently silenced mastodon.social, which again is mostly about calming down the local discovery features (you can still follow/be followed). They are blocking instances that encourage copyright violations, etc.
If you want an .art account, IMO right now you should start on one of the other instances they recommend, and move your account later. You can browse the local .art timeline and their view of hashtags without an account.