From a technical standpoint, there's little difference between instances. They're all running the same software at their core, and connected to the same network of websites.

Some instances, such as Beehaw, seem to strive more for building an actual community around their instances, while other instances aren't as interested in that and are focused more on just being another available alternative entry point to the Fediverse. I've not looked around Beehaw too much, but from what I understand, they seem to want to create a highly-positive environment for their users (hence why they've had to temporarily defederate from some instances, until there are better moderation tools available for them to keep trouble-making outsiders away from their community).

Some instances may focus more around a particular political leaning, some may be specific to a hobby like video games, etc. But at the end of the day, there's nothing stopping you from registering to as many or as few instances as you want. For the most part, they're all connected to the same overall network, so you'll typically be seeing the same content no matter which instance you browse from.

Keep in mind that your username is going to be based on the instance you sign up with. For example, I'm Chozo@kbin.social, and while Kbin and most instances will just show that as "Chozo", my account is still permanently tied to kbin.social, even if I'm posting on threads from lemmy.world like this one.

I think your best bet is just going to pick whichever feels best for you in the moment. I think for most users, it doesn't really matter. If you're looking for a specific community, then check 'em all out and see which one fits your needs best. If you don't care and you just want something that works, then any of the larger instances are probably going to be sufficient for you.