I just took the SFO flight last night and service was good. I was upper deck window and there were only 15 pax upstairs. catering/service was basically the same as LAX. SFO is no doubt a better terminal, but nothing special.

My 1 really bad experience on CX was on an HKG-LAX flight on the lower deck. crew was rude, chaotic and skipped several rows for dinner so everyone had to ring for the FA's.

In my opinion the difference in service is not LAX or SFO but rather upper deck or lower deck. I just think the service is more relaxed upstairs.

Just use whichever airport fits your schedule/budget (SFO flights in full J are $2K cheaper) and LAX flights have a higher chance of technical stops.