I would recommend Koray's place because the place is really strategic and convenient. The Osmanbey Station is just 10 minutes away and there is a mini supermarket called Sok just underneath the apartment. The drinking water is just 0,40 TL so we made sure to drop by this supermarket everyday. Sisli area has a lot to offer. You can find restaurants and places to shop along the streets. Atarturk Muzesi is also situated in here and you can just go there for free. Koray is such a great host. He is so responsive and helped us a lot during the trip. I dont have any problem finding and checking into the place as the information given is really accurate. I would recommend you to stay here while you are in Istanbul.