I think you may be a little biased, haha.
Anyways, if you are going to strictly drag race, the easy winner is the GTO. However if you want to have decent handling balanced with good performance, I would go STI. I have not driven either, but I think both are really good cars. You also may want to consider seating space, you will be able to drive around a few friends if you get the STI, whereas the GTO is only a 2 door. I was behind a red GTO on the way to work yesterday morning through the Liberty tubes and I must say that V8 sound amazing (I think he had a full exhaust setup though).