The best location hotel should be Bell Tower Hotel which is located at the city-center and you could see the nightview of Xi'an well. But, since close to street, a bit noisy at night.
For another choice is the 5-star Hyatt Hotel (very comfortable) which is located on the prosperous East Street, and just 10~15 minutes to the city center on foot.

For one-day tour, of course go to see the Terra-cotta Warriors and Horses. If time left, please go to Shaanxi History Museum and then the Big Wild Goose Pagoda, both of which are located very close to each other.

I stayed at the Sofitel Hotel on Remin Square when I was in Xi'an June 2009. 5-star hotel, a little more expensive especially the restaurants in hotel. Highly recommend hotel for comfort and location. I suggest you try to visit Muslim Street area for local cultural flavor and great food. If you only have 1 day, I think your time would be best used staying within the city wall. Plenty to see without having to rush to more remote locations. Have fun. I will be back in Xi'an end of Sept beginning of Oct for Chinese National Holiday.