Gotta go with a trailer on this one. I think cargo bikes are cool, but for me that facilitates having at least one other "fun" bike, but purely utilitarian people could definitely get by. Add to that bringing it upstairs and storing it seems nigh on impossible, and SF theft is pretty high, so locking it on the street should be a no go... Get a nice trailer suited to your needs. You can unhook it, bring the bike up while trailer's locked on the street, then come back for it... Many will fit in a small corner of a closet or something. Have front or rear rack on the bike for odds and ends and light hauling needs, and only pull out the trailer when you need it.