I would recommend United airlines.

I chose UA because when downloading the PDF master schedules from all the airlines, they had lots of flights from SFO to places I wanted to go.

After flying 25K miles and becoming "Premier" (the lowly "2P" level of status), I found the benefits very good. You get to fly economy plus which on some routes gives you a lot more leg room in coach.

While upgrades are reportedly hard to get for 2P members, I was once offered an upgrade to C class when flying to JFK for $150. It was only my third ever Business class trip, but I found it considerably better than either of the other two. (Essentially they fly international-like planes between SFO/LAX and JFK.)