I traded in my 2004 STi for a 6 speed 2005 GTO
I thought I would share my feelings
Interior - The GTO's interior is much nicer. The seats (which are excellent), CD player, over all look, etc. The interior is just as nice as my moms
Overall driving experience. The GTO is much smoother. It handles bumps very smoothly and just drives extremely nice. It is a very comfortable car. My STi was very hard sprung. You felt every bump in the road. It also rattled and made a lot of noises. Overall diving experience is much nicer in the GTO. And the seats are just so comfortable on the GTO it really makes the car a pleasure to drive.
Performance - My STI felt faster 0-60mph (and most likey was). I think its due to the STi's very short gearing (4th gear shift by 75mph) and awd. The GTO on the other hand is a little difficult to get off the line fast. It wants to spin the tires and its extra weight takes it a second to get going. Above 80mph the GTO is just flat out faster. I would expect a 1/4 mile race to be very close. The STi would definately get the jump off the line. The GTO would most likely catch the STi at about 80-90mph. Then the GTO would just pull away hard at the end of the 1/4 mile. At speeds over 100mph the GTO will just leave a stock STi in the dust.
Handling - The GTO handles really well (suprisingly well). It holds a turn really well, stops quickly, and the stearing is tight. Overall handling is really nice. But my STi handled like a go cart. The tight supension, sticky tires, and awd, made it handle better than anything I've ever driven. Thats hard to beat. It would tear up the GTO on a tight road course. But the GTO wasn't made to race on a tight course. Its made for open roads where it wouldn't have much trouble at all putting distance on an STi.
The shifter is a little sloppy and the throws are very long (compared to my STi) You waste a lot of time with the shifter. A better shifter would probably shave several tenths off a 1/4 mile time.
So over all I love the car. Each car does what it was designed for really well. It all depends on what your looking for. But I'm very happy with the car.