I stayed at both hotels in mid march (using the city of magic promotion). I really enjoyed both hotels, my first time at the FS Bosphorus and third time at Sultanahmet. Sultanahmet is much more conveniently located to see the historic sights versus the Bosphorus FS. You can literally walk out the front door of the FS Sultanahmet and turn the corner and the Hagia Sophia, Topkapi Palace and Blue Mosque are right there. It is probaby a 15 minute cab ride from the FS Bosphorus to this area. I would split the stay between the 2 resorts. Stay at Sultanahmet for as many days as you think you will want to see the sights in this area and then move to the Bosphorus to use as your base for exploring other areas of the city. Whatever choice you make, you won't be disappointed, both are great hotels which will take care of you during your stay here.