Engine: 245 hp, 230 lb-ft of torque, 3.0L, 24-valve DOHC V6
Curb Weight: 1,705 kg (3,759 lbs)
IS 204 HP 185 lb/ft torque
Curb Weight 1,656 kg (3650 lbs)
There is a big price difference. Only a test drive will tell you if its worth it but this jumps out at me.
Both are heavy cars especially with AWD and Auto only tranny's. The GS has a better power to wight ratio and a substantial torque advantage.
I don't know how important it is to you but the IS 250 AWD quoted 0-60 is 8.3 seconds. It means you'll get smoked at stoplights by Mazda 3s. For a "sporty" BMW 3 series competitior that would simply be unaccepatable to me.
Nice dillema to have Gs or IS.