sub-domains or sub-directories??

OK, I am trying to set up a site like Etsy.
I have the multisite plugin- I enabled wildcards

Now working on Network:
I am trying to set up a network I'm not undersanding the difference between sub-domains and sub-directories. What should I use. It says I can't change it later.

I have paypal connected to another account, can I use it for this account too, or do I need to set up something different?