1. 6 hours is not enough time to have any real experience in the City. Figure on 1 hour for taxi, "deplaning" and inter-terminal transfer, to/from Airport/City, 2 hours before departure to check-in and to enjoy a drink or two in the lounge and you're talking only 3 hours left. As for dinner, SFO itself has some really nice restaurants in the International Terminal. But I wouldn't have a full dinner, see next answer.

2. Full dinner service.

3. J seats are at an angle. Some like it, some don't. But service is one of the best.

4. Just about the same.

5. Lounge experience is a must at HKG. The Wing and The Pier are reputedly the best lounges in the World. Pity it's not there where you have the 6 hour layover.