EVA premium economy is not the same as UA premium economy. It is sold as a separate class. Usually L or V. B is just ordinary economy. If you aren't booked in Elite Class/Deluxe class you can offer to pay for an upgrade at check-in but these days with the 777's it usually goes out full so if you didn't buy an Elite Class/Deluxe class ticket you have pretty slim chances of getting upgraded by any means.

As for EVA vs China Airlines? You will generate some controversy there . I prefer EVA but would have no hesitation about flying CI either if that were somehow beneficial to be. Others will offer the opposite opinion. They are both fine.

As for SFO vs LAX on EVA? I believe they are still flying the old 747 Combi's out of SFO. LAX is almost exclusively new 777's now. This is relevant because the economy seats are better in the 777's due to the AVOD whereas the 747 Combi's have dumpy old ceiling TV's in economy.