If you just choose one of the general servers from those two sites, you should be okay.
All of the servers on those two sites have promised to uphold specific standards (listed at https://joinmastodon.org/covenant).
On Fedi.garden there's also the founding date listed, which gives you an idea of how long they've been going. If a server has been going for years, that's a good sign too.
And you can always move your account to another server if you don't like the one you chose.