It's much more convenient to fly into Ataturk Airport ( IST ). Ataturk Airport
is main, so more crowded and busy. But its in European side which have almost all touristic sides. It is accesible by subway called M1A Metro. Sabiha Gokcen Airport is in Asia side is very far from the main attractions in Istanbul and it is not accesible by subways. Trying to get to the European side of the city from Sabiha Airport, if you are a first time visitor, is not easy. A company called HAVATAS or HAVABUS run by the Istanbul Municpality handles transport to and from both airports.