A taxis from Sabiha Gokcen to Sirkeci will cost between 90tl and 110tl depending on traffic and, if the bridge is busy, can take up to 1:30mins.
The Havataş is 14tl and drops in Taksim Square, same as taxis - with traffic it can be up to 1:30mins. As Colwyn says, you can then either get a taxi direct to your hotel or metro to Kabataş then tram.
In my opinion, thats a lot of fannying around when you can jump in a taxi and go straight to your hotel for less than £25.
An alternative though, if you don't mind transferring, is to get the Havataş to Kadıköy (9tl) then get the ferry to Emininou. Cross the road and 100m to your left is Sirkeci train station. The ferry is 4tl per person, but if you buy an IstabulKart (the old Akbil) at the airport it works out at 2.5tl, or something like that.
JF