I really loved my time at Nancy's house in Missions District.
First, Nancy is a great host, who you can easily chat with and who has great recommendations for you.
Second, there is Satchmo the cat, who loves to meet new people and is really cute.
Third the house is a very beautiful old place with lots of vintage objects that tell their own little stories. It is situated in a very vibrant neighborhood with lots of good restaurants and interesting shops.
Last but not least it is very easy to go downtown, because there is a BART station in walking distance and several bus stations too.
All in all I had a great time in SFO and at Nancy's house, so I can only recommend staying there.