I believe that even with commuting to SFO you will have more days home at UAL. UAL you can trade, bid, and manipulate your schedule to give yourself commutable trips. You will gain seniority at a much faster rate at UAL, and as you mentioned the opportunity for widebody flying and more pay.
SFO is a relatively junior base, and my buddy who has been at UAL 3 years can hold SFO 777 or 787.