We were just in Xi'an. It is much bigger than we thought and transportation is not as easy as larger places like Beijing and Shanghai where there is a subway that takes you everywhere. We hired a private guide who arranged for an airport pick up and drop of f and to take us around Xi'an. If this works for your budget, I would certainly recommend this. We arrived at night and has 2 days of touring. We went to the Terra cotta warriors in the morning, the Neolithic village (Bampo people) afteward, and the wall around 4 pm. We were on the wall for a short period of time as it was later in the day (we were a little tired) and a little cool there. It cost 40 RMB/person and not so worthwhile as we were there for such a short time. In the evening, we went to the Tang Dynasty show that included a dumpling dinner. I think you can go to the show without the dinner. I would suggest to eat elsewhere and only go to the show (the dumpling were only okay). I am not sure of the difference in cost. The next day we went to the museum and it was hard to see the exhibits as it was so crowded. Since we paid the 20 RMB to enter, we were able to see a special exhibit which is not available if you go in for free and that was good (not crowded at all) . Afterwards we went to Big Goose Pagoda and the Muslin market. I was not into shopping and I am not sure how this is different from other markets. Then we went off to the airport.
If I had it to do overagain, I would stay 2 nights and leave early on the 3rd day and have 1 day of touring - see the Terracotta warriors in the am, Museum in the afternoon, and Tang Dynasty show in the evening.
Certainly worth stopping in Xi'an