I still can't decide between Double Tree Old Town or Hilton Bosphorus. It's my first visit to Istanbul and I will be there for 5 nights.
My two cents
here, in the general "Hilton HHonors Properties in Istanbul" thread, where you will also find more "which property?" discussion.
Short version of my opinion: Doubletree if you like being in the middle of the old stuff, Hilton if you like new stuff, a city feel, and nightlife. Personally I feel that there is much more to Istanbul than the historical sites so I prefer the latter.