It takes over 1 hour by cab from the airport to Terra Cotta Warriors. It takes about 40-50 minutes by airport bus, more or less the same by cab, from airport to the city center. If you only have 1 or 2 hours only, I think it is not necessary to visit.

what is there to do in xian center city? are there any 5 star hotels?

there are several 5 stars hotels in Xian,Sofitel is very good

The city center of Xian is the Bell Tower, where you could take photos and then walk to the nearby Drum Tower. At the south side of this tower, you will find a long street with many kinds of local snacks and muslim restaurant. If time possible, you could walk northwards from the Bell Tower to visit the City Wall. Only takes you no more than 10 minutes to arrive at the foot of the wall.

It is easy to find a 5-star hotel in Xian, such as the Hyatt Regency, which is only about 2km away from the city center.