They pretty much the same thing. But if you are going to offer all of or some percentage of the products like T-Shirts, Towels, etc, etc, you can pretty much ignore FineArtAmerica and focus on Pixels. Actually you need to focus on your ArtistWebsite if you paid for the Premium Service.
FineArtAmerica site does not show all the products. Pixels shows everything.
There are no rules about that. Some are more comfortable with FAA and others with Pixels. When I joined this site there was no Pixels site and no merchandise for sale, so I'm more comfortable with FAA.
Anyway whatever work you upload to one goes to the other.
if you paid to be here, use your artist website. i use fine art because i like the name better, but i advertise my AW. others like pixels but overall they are the same site, FAA has less stuff in it. you'll find decor items in FAA.