May 2014 - Turkey
The answer is it depends on what you are planning on doing in Istanbul. If you want to see all the major sites of the city such as the Blue Mosque, Hagia Sofia, Topkapi Palace, the Basilica Cistern and the Grand Bazaar then Sultanahmet is the property for you. Avoid the traffic and all your need is your two feet to see it all. The Hotel is stunning, quaint and small. The terrace on the forth floor has amazing views of the Blue Mosque and Hagia Sofia, well worth a drink (or many).
However if your visit to Istanbul is less touristy and more to the nightlife and shopping then Bosphorus is the place for you. Due to the steep hills around the city many places will still require a cab, but a 10-15 drive is better than an hour from the old city. It is also the place for chilling out, whether cruising the Bosphorus or swimming in their spectacular pool or eating the night away by the water. The palace grounds are spectacular and pulling up to the Hotel in a boat is an experience that cannot be missed if you are fortunate enough to stay here.
Bosphorus is the bigger property and you can tell by the service. At Sultanahmet everyone knew within the few hours of our stay who we were and even when we went back to the Hotel a few days later (when we were staying at Bosphorus) the staff at the door and front desk remembered us by name. Kudos to them all.
But then again if you don't have to make a choice don't. Stay at both if you can, three nights each or two and three depending on your preference to siteseeing or relaxing/nightlife. One night is not enough to enjoy either property.
I love when someone at a FS says this to me. So simple and yet fills me with such warmth.