Nicer by a Nose
I just completed a two night stay for business in San Francisco, and spent one night at the Hilton San Francisco Financial District and one night at Westin San Francisco Market Street. I think the Hilton wins by a nose,but it was a close call.
On the location side, the Westin is a bit more convenient for my purposes. Both are walkable from the BART, but the Westin was much closer. On the other hand, the Hilton is pretty much in Chinatown, which made for some interesting dinner choices right outside the door.
Check in took a while- the lobby was busy, and the line moved slowly. I am a Diamond member of the Hilton program, but it did not make much of a difference checking in, or with the room. I was hoping for some sort of upgrade, but all I could get was a regular room on a lower floor with a view of brick walls and the alley. I did get free internet access, and access to the lounge.
The lounge was not as nice as the Marriott or some of the Sheraton lounges, which usually feature some sort of hot breakfast choice. The venue was ok, though small. Breakfast consisted of breads and pastries. The espresso machine was broken, and they kept running out of coffee, but the lounge attendant was friendly. 
The gym was very small, but had a lot of equipment crammed in. It did include a few weight machines, though not a full circuit.
Overall, the hotel felt clean and the room, though small., felt new.