Hi,
I have only flown on the 773 btw HKG and LAX/JFK (3 separate trips) in Y...for what its worth, it was fine in coach. As an EXP, I was able to call CX and pre-book an exit row aisle seat (row 31 I believe). CX was even able to block the middle seat for me, which was key on the long flights.
As for SFO vs. LAX...One of the advantages of LAX is the availability of the Flagship Lounge (assuming your EXP) once you arrive...its great to take a shower there after the long flight. I also thought the staff was excellent.