Currently in Xian and found a lot of info online is out of date and incorrect.
Airport Express - Bus route numbers have changed and buses don't even display the number. There are six lines and you need to check each stop (route in both Chinese and English). I took #5 to get to the railway station (not #2 as found online). Fare is 26 yuan and sold by stop for #6. Chinese website of bus operator cwagi.com
Bus for Terracotta Warriors. I took bus 306(5) from southeast of railway station. This bus uses the expressway and has many stops for locals. It took just over an hour and cost was 7 Yuan. First bus at 7am and ticket office for the warriors opens at 8:30am
Returning, I tried bus 914 since there were about 5 buses in the parking lot, it was written Xian Railway Station<->Terracotta Warriors on the side in English and one bus was just leaving when I showed up. Fare was 8 Yuan. Big mistake. The bus was fairly empty when I boarded and stopped and every transfer point picking up more passengers even when all seats were occupied. It honked at every pedestrian on the road trying to pick them up and didn't use the expressway. I took almost two hours to get back. Avoid if possible.Edited: 11:17 am, May 12, 2013