Here is the answer I got on PowerSeller US (if somebody would like to know)
They're intentionally misleading if you ask me:
Featured Plus will place your listing at the top of search results that find it. So when you pay your $20 to add Featured Plus to "NICE OLD ETRUSCAN BOOK L@@K", your auction will appear at the top of the list whenever somebody searches for
Etruscan -arrows +book
Gallery Featured will increase the size of the gallery picture, and really only effects the auction for people who are browsing the gallery view. Since (a) the Gallery has recently been dead about 10% of the time, (b) nobody in my experience ever browses the Gallery instead of searching for keywords, and (c) it's a flippin' waste of money... or, yeah... therefore it's a flippin' waste of money.