From my experience last week I would say the Westin is in a better location for tourists as you can walk to either subway stop and for less than .50 cents get anywhere within central Beijing.. including the bus stop that takes you out to the great wall of China. It is however not within walking distance of any of the major sites unless you think a few miles is walking distance
The great wall location however is a taxi drive to everything except for the few bars that many of the expats and tourists visit such as Hard Rock and Durty Nellie's.. so if fun bars are your goal of touring then great wall is better..
Let me know if you have any other questions I can try to help you with as I was just in Beijing last week and definitely enjoyed my room and the lounge at the Westin.. The spa is also supposed to open on the 22nd which will be a great addition to this hotel.