No choice doesn't matterat all. However, the decision on which mastodon server to use for your social media is about as important as what you'll choose to eat today for dinner. Yeah, kinda important for the dinner itself and you don't want some crap, but if you do, you could just eat it anyway for now and try something else tomorrow.
When you pick "federated" you'll see all posts, independent from your instance. But that's pretty much impossible because unless you have Tusky the posts will be too fast.
So yeah, to be able to read anything you have to just read the posts on your instance, meaning it does matter which one you pick.
It's not even the speed. It's also the content. All from all instances seems cool until you can't read 80% of it because it's in another language. I'm glad those instances exist but when I have to scroll through 5 posts to see one I can read, and it's some low-effort post (on average) the experience is not a good one
Even after all that work, you've gotten through 3 seconds of the world's timeline
Local is a good one, if your instance is lively enough. Then you can add people and see boosts and your world grows nicely
I find the "explore" page super useful for exploration! Spend a few minutes twice a day or so and the content quality is quite high, great place to branch out into boosts from. On cryptodon, I also do my best to curate the global feed to be somewhat useful (though things to creep in at the edges, I need to do some culling now for some Misskey bridges that are pretty loud in that feed).
There is no functional account migration, therefore your choice matter unless you think what you write doesn't matter, in which case, why write at all ?
It's very long standing issue, first issue was closed as completed but not complete
Apologies, my familiarity with some of the terms of Mastodon are still a bit rough around the edges. By relationships, do you mean your followers and who you follow?
If so, it does seem like your followers is preserved (so long as nothing inhibits the move signal), as for who you follow its not clear on the docs whether that is preserved or not (and I've not done an account move of my own) - but you can export a list of who you're following from your account preferences, and then reimport it on your new account for sure.