Hi! I go to UC Berkeley and my fiance is at Davis so I'm really familiar with the Davis-Berkeley situation. I would definitely recommend Sac if it isn't overly expensive, because Amtrak can be pretty pricey and the combo of Bart to Amtrak + Amtrak to Davis would definitely be a hassle. If you're still set on flying into the Bay, I would recommend checking out the Oakland airport, it's a much easier destination (in my opinion) if you're getting to the East Bay, which you would be to get on the Amtrak to Davis.