I was just going through this with some other
domains this weekend. I think it's always safer
to go with the shorter singular name. The plural
probably loses more traffic to the singular than
vice versa. Still, if it's a product that people
normally buy more than one of, 'books' 'cards'
'decorations', then the plural is maybe equally good.
The reason I prefer the singular is that it's shorter,
and they have to type in the singular to get to the
plural, while the reverse can't be said.