It's our first visit to San Francisco, we are coming from South America, we would like to Know which of the following areas is better to stay? South of Market, Union Square or Nob Hill!

South of Market - spotty. You need to KNOW the exact hotel so not recommended for those not familiar with the area.

Nob Hill - Very POSH. Some of the most expensive hotels in California are on Nob Hill (Mark Hopkins, Stanford Court, Fairmont, etc)

Union Square - very convenient and the best shopping area. Price Ranges from moderate to very expensive. You will find a lot more to choose from in the 5 or 6 blocks around Union Sq.

Of the three areas you mention, Union Square would probably be the best from a tourist standpoint. I think it is more conveniently located as to the tourist attractions. Nob Hill is a nice area, more upscale, but not as convenient. I find the South of Market area kind of away from everything. You didn't mention what you wanted to do or see in San Francisco and that might have some bearing on where you want to stay.

I would never recommend South of Market (SOMA) as a place to stay. It's becoming more and more interesting as an area for restaurants and clubs, but still a bit iffy if you don't know the city. Nob Hill is a steep climb up or down to get to any of the tourist destinations. And, as romantic as the cable cars sound, they are a pain when you want to get somewhere in a hurry. If you stay near Union Square, it is possible to walk all the way to Fisherman's Wharf on fairly flat streets going down by the Financial District and then along the Embarcadero.

Thanks a lot for your advices, they were of great help. And what about pacific heighs, cow hollow an the marina? I've found some hotels there recommended by fodors.

Pacific Heights is lovely but is again rather out of the way if you want to walk to things. Cow Hollow--you need to narrow this neighborhood down. If you mean Union and Filbert Streets from Van Ness to Steiner, it's fun and trendy and a great place to walk. Lots of little shops and restaurants. If you mean hotels on "motel row" on Lombard Street from Hyde Street to the west, it's perfectly safe but can be noisy at night because of the traffic. If you're a light sleeper, that might bother you since many hotels don't have A/C. I think the Marina is isolated and a better place to live than to stay as a tourist. A lot of this might be determined by your budget. Have a look at www.sfgate.com and click on travel.