Congrats on the CJO. I had offers from both companies and am comfortable with my decision.
The commute from SEA to SFO isn't fun but it's worth the seniority progression due to mandatory retirements and about 3% growth. Someday when you get to a widebody, the commute should get easier as it's afternoon/evening departures and you're only doing it 3 times a month.
SFO is united's junior base. You will see great progress....I've been here nine months and am 50% in my catagory. I bid for three work groups...so I do a 5/6/4 day trips in July for example to minimize the commute.