Among the SPG offerings, the Westin would be my choice since it's next to Large Wild Goose Pagoda (I like). The drawback is that it's kinda far away if you want to get to Drum/Bell Tower, although taxi or bus can get you there. The surrounding area of Westin is more upscale and there's many more international restaurants, as well as full Chinese spectrum including fine dining, noodles of all kinds, street barbeque, ruo jia mo, fried bread, etc.

Sheraton Xian is decent too, Same issues with location outside wall and far away from Bell tower. Surrounding area there is less upscale, but I still found a famous dumpling restaurant, a barbeque area, ruo jia mo, soup noodles and massage all close by within walking distance. Need taxi or bus to go into the wall. Has great breakfast and lounge, probably better than Westin.

There are good western hotels in other areas too. Sofitel and Hilton both have good properties in good locations. Can't comment on Sheraton North City, Shangrila or Crowne Plaza. All of those are outside the wall.

I would agree that for the taxis, it is sometimes challenging to return to the hotel. There is language issue if you can't speak any Chinese - learn how to say the hotel name and the street name of the hotel (as well as north, south, east and west road if that applies). A hotel card is hit and miss for return. I agree that in Xian it's a benefit to know how to use the bus routes. There are times when you will not be able to get a taxi (shift change, rain, busy times on busy streets). You will see streams of busses passing you by as you wait for a taxi. Learn a few of the routes and you will be thankful.