Do you prefer Signature? If you do, Petaluma is the way to go as they are the only "Signature" outlet in Nor. Cal. Napa is smaller I think, & will have minimal Signature (store returns & maybe some accessories).
The Outlet Mall at Petaluma I like better than the one in Napa. There is an Off 5th Avenue (Saks) outlet right next to the Coach store in Petaluma.
Actually, even if you like Leather better (which I do), Petaluma might still be the way to go as it's closer and they also have Leather. The Saks outlet also carries Coach,
Prada, etc.
This will give you the list of stores at Petaluma:
http://www.outletbound.com/cgi-bin/...CATED 50 MINUTES NORTH OF SAN FRANCISCO&ssr=3
and at Napa:
http://www.outletbound.com/cgi-bin/...A|LOCATED 1 HOUR NORTH OF SAN FRANCISCO&ssr=3
Oh, and I think Gilroy is even further than Napa or Petaluma. If you want to go to an outlet larger than Napa that is primarily leather, I think Vacaville would probably be closer than Gilroy. Ok, I decided to MapQuest for you - 1hr 20min. to Gilroy, 58 min. to Vacaville, 55 min. to Napa, 50 min. to Petaluma. Now Petaluma isn't only 5 minutes from Napa, but it's because you take a different route to get to each that the time works that way. I still think Petaluma because the drive is so pretty.