Los Angeles and San Francisco are both large and populated cities (Seattle in particular), so traffic conditions can affect the drive time on either end. With optimal traffic, taking the I-5, you should be able to make it in three and a half hours without stops.
The usual pros and cons of driving all apply; such as environmental impact and lower cost (especially when using MirrorTrip) which both weigh in favor of driving. Normally, speed is the main advantage to flying however in this case I think given the short duration of the trip, you're probably likely to spend more time waiting around in airports than you would on the road.