I've only stayed at the PH (last December) and found it to be wonderful. However, having grown up in the city I can offer some perspective. While it will take you a good deal of time to get into the PH or the GH from the airport, it might be worth staying downtown since you do have a fairly solid chunk of time in the city. The area around the PH, called Nisantasi, is a trendy neighborhood with many restaurants and stores, mostly frequented by locals. An interesting side of Istanbul if you have not experienced the new city. The GH is located close to Taksim Square, a central area that is less interesting and that has been losing prominence among tourists and locals alike in recent years. The GH is also a much older property; the rooms at the PH are far superior. The Regency is close to the airport (about 15 minutes) but there is next to nothing around it. The ferries that you can catch from nearby will only take you to the Asian side, which can be interesting but I would prefer Nisantasi for sure. The Regency is quite new and the rooms look beautiful, and you might prefer it to the GH from a hotel-only perspective. However, the hassle of getting into the city by taxi (or taking a taxi to the nearest metro/tram station) is not worth it, in my opinion, if other factors like cost are not critical (the HR is usually significantly cheaper than the PH and the GH). Hope this is helpful!