Well...Napa is probably comparable or lower than Walnut Creek. There is more variety in housing stock, too, with a significant Victorian/Craftsman era historic district. Napa is not as homogeneously affluent as suburban Walnut Creek (it's a somewhat working class town, actually). The job market is not that strong, though the opening might be interesting. I like Napa a lot. The climate is nice (cooler than Contra Costa County in the city), I like wine snobbery and good restaurants, and the countryside is extremeley beautiful and very diverse (rolling vineyards and sloughs to the south, agriculture to the north in the valley, redowwod forests to the west, and dry but beautiful oaks and chapparal to the east. It's an amazing place.
Petaluma is a little more expensive because they have growth control and pretty high standards for the new suburbs. The town has a littl more "going on" downtown (although Napa actually has a Mervyn's Department store and a Santa-Barabra style paseo mall that has always struggled) . Petaluma is an old agriculture center, so it is not completely polished-there is more gritty agribusiness stuff around, including big grain siols downtown. I think this adds character to the town myself.. There are also more jobs and a stronger economy, overall. Petaluma is a short distance to other jobs centers, including Santa Rosa. Petaluma's setting is not quite as scenic (but still pretty, just not as varied), and the dairy farming in the surrounding lands can be...fragrant
Both of these options are very car-dependent, although both towns have downtowns that are worth living near and walking to for boutique shopping and
Good luck!