I stayed at both the Conrad and Hilton Istanbul in December 2012. Both were perfectly fine, although of the two the Conrad in particular is showing it's age. Service at both was first class.

The Exec Lounge at the Conrad has stunning views across the Bosphorus. It's a relatively small lounge, but I found it fine for breakfast and drinks in the evening. They are currently renovating the rooms, so if you can get a newly renovated room I would recommend doing so. My room was not renovated, and whilst perfectly comfortable and clean, it was nothing particularly flash.

The Hilton is closer to the centre of town for sightseeing, and was an enjoyable walk from the hotel to the old town. It also has pleasant views out across the Bosphorus, but the Conrad is blessed with a superior and unobstructed vista. The rooms are more up to date, but they are not cutting edge either.

Enjoy your visit, Istanbul is a spectacular city and I cannot wait to return.