I would never take an organized tour in China (or anywhere else, for that matter) -- all they do is insulate you from the culture you've traveled all that way to visit.
A taxi to Xi'an is by far the best choice and not hard to negotiate. Your hotel can assist you if you're reluctant to initiate the transaction. Note that there are other interesting things to see in the area around the terracotta warriors, including the Qin Emperor's tomb (a small man-made mountain, not yet opened, but interesting -- at least to me -- none the less). The terracotta warriors are particularly well-curated. There are three large halls that show various stages of the excavation, as well as a very nice museum. All of it is extremely accessible to foreigners (English signs are everywhere), and no tour guide is needed. Note that you will run two gauntlets -- one of vendors of tourist souvenirs and one of tour guides. Politely, but firmly, saying "Bu ya" (don't want) will get you through.
Also note that there is a lot to see in Xi'an besides the warriors. Our favorite sights were the Little Goose Pagoda (older and more atmospheric than the Big Goose Pagoda), and the ancient Buddhist mosque that is still in use. The mosque is in the middle of a large Moslem neighborhood which, in itself, is very interesting to see.
China has also opened a large cultural park dedicated to the history of the area.
We were there for ten days in January, and didn't exhaust everything we wanted to see and do in the area.