Do you mean lay-over or connection? Layover can mean many hours, even an overnight. If your onward flight is within a few hours, it is a connection.
In either case, it sort of depends on what time of year you are traveling. SFO can experience delays during the fog season or periods of bad weather.
You have actually asked multiple questions. For nicer and easier, I'd pick SFO every time. For less likelihood of delays, LAX. For most of my flights to Asia over the last 10 years I have connected through SFO but will use either depending on the best flight costs and times. I prefer SFO but not to the extent of paying more or flying at inconvenient times or on sub-par equipment.