Charles M. Schulz Sonoma County Airport, Santa Rosa: This is the sensible airport if its limited nonstop network works for you. Rent a car at the airport and drive west via Santa Rosa/River Road: roughly 30–40 minutes. A taxi/rideshare is about 26 minutes and roughly $50–65, but do not rely on rideshare for the return trip. Sonoma County Transit is cheap but takes about 2½ hours and is not a good arrival transfer with luggage.
San Francisco International Airport: Rent a car and drive north on US-101, then take River Road west beyond Santa Rosa; allow about 2½ hours in clear traffic and considerably longer on Friday afternoons, summer weekends, and holiday departures. There is no clean rail-plus-shuttle transfer worth recommending: Guerneville is a car destination, not a transit extension of San Francisco.