The Lexus ISF is faster.
The Lexus GS350 Fsport has a 306 HP 3.5L V6 engine with 277 ft-lb of torque.
The Lexus ISF has a 414 HP 5.0L V8 engine with 371 ft-lb of torque.
Lexus ISF is a performance car that does not compete with a mid-size family car such as the GS. Note that Lexus is currently developing the upcoming GSF.
The Lexus ISF competes with Audi RS4/RS5, Mercedes C63 AMG, BMW M3, Aston Martin V8 Vantage, Porsche 911 Carrera S, Audi R8 4.2L V8, Maserati GranTurismo.
The Lexus GS Fsport competes with BMW 550i, Audi S6, Mercedes E550, Jaguar XF, Cadillac CTS Vsport, Lincoln MKZ.