Rozi hasn't posted on FT for quite a long time.
Personally if I were coming to IStanbul, Neither the Conrad nor the Hilton would be in the top part of my list. The top 4 would be (not necessarily in order) The Ciragan Palace (Kempenski), The Four Seasons, The Ritz Carlton or The InterContinental. These are all much better properties.
As far as the Hilton and Conrads go, let me put it this way. I know literally dozend of people that have stayed at these properties over the years. People from different countries, backgrounds, etc. Also living in Istanbul, I have to frequent these properties from time to time for various reasons (Played tennis at the Conrad just the other day as a matter of fact, and was at a wedding last month at the Hilton).
No one particularly likes the Conrad. Most people complain about high prices, mediocre rooms and bad service. Since the Hilton was last renovated, I hear no complaints, and service has never really been a problem from what I've heard.
The Conrad is a newer property, and while both would be classified as near eachother in a geographical sense, they can be 20 minutes apart in "normal" daily IStanbul traffic. The Hilton is much more central anasmuch as it is near Taksim square, and therefore by definition is more easily accessible. While the Conrad is not that far away from things, it is the large Avenue, Barbaros Blvd. that basically screws it up.