imho both Hiltons are in rather bad places when you want to go sightseeing, especially the hill area (Dr. Sun Yat-Sen Mausoleum etc.) including Presidential Palace, Nanjing Museum - all are located rather on the East of the city.
If you want to visit the Memorial Hall of the Victims of the Nanjing massacre (Warning, though, it's not for everyones heart, albeit "top rated" from TA, I could take it, and it should imho be mandatory for everyone to visit in their school time, especially for japanese) the Hilton will be perfectly located. You might lose a bit of appetite after the visit, though.
The Fairmont is even more far away from the city - very much in the south west of the city. It's nearby a good metro interchange station to it's not that bad traffic-wise (especially when you arrive/depart at South Railway Station) but if you go into the city centre it's about 20min door to door at least, and to the "attractions" in the east another 20min or so..
The Metropark Hotel (
http://www.metroparkhotels.com/hotel...?hotel=nanjing) which used to be a Hilton till a few years ago is in pretty much ideal location for much of the attractions on the eastern part of the city, including those on the hilly area outside. I know it's not a Hilton anymore (and therefore no benefits for you) but just a suggestion of an alternative.
So, if you're more interested in the city itself, I would say: Hilton (city) > Fairmont > Hilton Riverside. If you're interested in the historical sites / Museums of the East, I would look for other hotels, but if it's down to those 3, then the Hilton would be the best bet.