I was just in SD in February and had the same issue. I ended up staying at the Hampton Inn Downtown. The location is right behind the trolley stop and is a 10-15 minute walk to the Convention center. I walked both directions at night by myself-its extremely safe. If you intend to stay in the city and meet others for dinner or drinks, I suggest you stay here as you can walk or take the trolley to anywhere downtown within 10-15 minutes. The Doubletree Mission Valley is much nicer but also more expensive and a longer trolley ride (30-35 minutes) to the convention center. The facilities, rooms, and pool are much nicer at the Doubletree Mission Valley. One big warning for the Hampton Inn Downtown is the Amtrak and freight trains. Make sure you request a room facing the ocean. When you check in, confirm you have a room facing the ocean. The rooms facing the city or courtyard abut the train tracks and you'll be woken up every morning around 2:30 from the train horns.