SFO (or any other US airports) - unnecessary US formalities and potentially long lines for transit on return; a hassle if not Canadian or US citizens (plus I am not a fan of UA).
PEK, PVG, CAN (or any Chinese airports) - uncontrollable delays within China - risk missing long haul connection on return.
The best options are HKG, TPE, ICN and NRT. CTU has limited international flights. So you should check layover time.