I'd second speedbird on the choice of the Fs or the Ciragan. Of the two, I'd say the Ciragan is by far the more authentic, and the view of the Bosphorous is superb. The FS doesn't have a pool, but is by the Hagia Sophia and Topkapi.

There are actually quite a few decent hotels in istanbul. If the two mentioned are full or too expensive, the Conrad, Swissotel or Hyatt are all pretty good, if not very central