@rufustheduck A few things to look for in instances:

What is the character count limit? This can be vastly different across instances. If you post long form content, this could matter to you.

What instances do they block? If you're into some stuff that others aren't into, and more power to you, check if they block instances with that stuff. Also check their content posting policy.

How good are they about updating mastodon? Are they stuck on an old version? How active is the admin?