Crochet Jewelry question. I want to make earrings. I am trying to figure out what is the difference between the thread sizes.?

There is no 'best'. You choose the thread that will produce the pattern in the size you want. If you look at a few patterns and compare the threads recommended, you should be able to figure out whether you want a large diameter thread or a finer one.

Go to the craft store and look at the materials or download a pattern. I don't know why you want thread, unless you are crocheting and adding beads. Just use your findings, as the wire is already shaped or use a wire jig, which you can get at the craft store. I have crochet thread, size 10, and I guess I could use it if I would make it and soak it in starch afterward as without that, it would be useless.

It's your choice. There is no rule.