First off, look at a map and you see that Napa and Sonoma Counties are right next to each other. They share a common border that's defined mostly by the Mayacamas Mountains. Actaully, they are "mountains" if you are from Iowa, but are "large bumps" if you are from Colorado.
|Image from napavalley.edu|
Getting to Napa and Sonoma
Either is a similar drive from the Bay Area, such as San Francisco or Oakland Airports.
Napa Valley is defined by one major road running the length of the valley, Highway 29, that goes from the town of Napa through Yountville, St. Helena, and Calistoga. There's a second road, the Silverado Trail, that parallels Highway 29 to the east. There are a number of crossroads between the two. This all makes navigation fairly easy. With only one main road there can be a lot of traffic to impede your travel.
Sonoma County has a freeway, US 101 that runs north-south. Otherwise, it's a mishmash of usually winding roads that occasionally change their name for no apparent reason. Expect to get lost, but that doesn't mean you won't run into new winery finds while you're lost.
Napa Valley is wine nirvana. You will drive right by Mondavi, Beringer, Heitz, and all those famous names associated with domestic wine. Napa Valley is the birthplace of American wine culture. Napa is is to wine geeks what Washington DC is to history geeks.
Sonoma will seem more casual, more laid back, not so stuffy. Napa can seem a little uppity about their wine and food.There's a lot of money floating around Napa and it shows.
|Western Sonoma County|
Image from mcclellandsdairy.com
You will spend more in Napa. In general, wine tasting fees and the bottles of wine will be significantly more expensive in Napa. Restaurants and lodging will be somewhat higher.
Napa Valley = Cabernet Sauvignon for a reason. Napa produces some damn fantastic Cabs. Sonoma, mostly because of micro-climates, has a much broader range of wines available. This doesn't mean you'll only find Cabernet at Napa wineries, just that you'll find a lot more variety in Sonoma. For instance, there are numerous Russian River Valley wineries that will make a dozen different Pinot Noirs. Ten miles away you'll find that Zinfandel is king. That's not something you'll find in Napa Valley.
Famous names, famous architecture, infamous crowds are all in Napa Valley. Though you'll find plenty of recognizable names in Sonoma, too, with folks like Kendall-Jackson, Chateau St. Jean, Kenwood, and Clos du Bois.
Napa has castles and palaces that double as wineries. Sonoma has warehouse wineries.
Napa Valley is a tough place to work on the front lines where you're dealing with hordes of visitors every day. People who were jazzed to move to Napa to follow their dream of getting in the wine business can suffer "Napa Valley burnout" after a couple summers. This means that sometimes people don't seem quite as friendly on the Napa side.
Yes, I've totally generalized here and maybe I'm a bit prejudiced towards Sonoma County cuz that's where I live. (I've worked at wineries in both places). Maybe it comes down to if you're a Disneyland person then Napa Valley is for you. If you're a Yosemite National Park person then Sonoma may be for you.
Since Napa and Sonoma Counties are right next door to each other check 'em both out. It doesn't have to be a "Napa OR Sonoma," it can be "Napa AND Sonoma."