I would highly recommend staying at Efe's apartment. Efe met me on the first day at Ayrılık Çeşmesi/Marmaray metro station, an easy commute from the airport and only 5 minutes from his apartment. He wrote to me during the week to make sure everything was going well. You'll find everything you need in Kadıköy (bars, restaurants, supermarkets). There are also many bakeries and coffee shops in the neighbourhood, and the ferries to the European side of the city are within short walking distance. Staying at Efe's apartment is a really good choice, especially if you want to discover more than just the most touristic attractions of Istanbul.