I also test drove a Liberty GT before deciding on the 6MPS. At the time of purchasing the 6MPS the Liberty GT was not actually being sold by Subaru as emission regulations killed off the 2.0L turbo engine at the end of 2005, and it was 6 months (maybe 12, I can't remember) before they released the 2.5L turbo. If I had wanted a 2.0L GT the only one I could find was a demo.
The link to the Wheels article that was posted earlier pretty much sums up my thoughts on the performance differences between the 6MPS and GT. I drove the 2.0 GT and 6MPS back to back at Tom Kerr in Sydney (and at Orange), and the step up in performance in the MPS when you hit boost was more aggressive, and once up and moving was significantly quicker in terms of in gear acceleration. The MPS pulled alot harder in the midrange than the GT, which felt more docile in comparison. Those characteristics are what sold me on the MPS. I've also driven the 2.5GT and while the low down torque improved it still didn't have the 'shove in the back' feel that you get from the MPS. I haven't driven the current Liberty.
I preferred the exterior look of the previous model Liberty compared to the 6MPS, and still do, although Im happy with the look of my car with the exterior mods I've done. I also liked the interior and dash layout more in the Liberty and the McIntosh stereo craps on the mediocre Bose unit in the MPS.
It's interesting reading people's posts on the Hypertech. While I feel the car has more power up top with the Hypertech installed (and it comes on boost slightly earlier), I have lost some of the mid range punch that I had pre-Hypertech. The power delivery is more progressive post-Hypertech, which I'm sure is better for the engine and drivetrain, but I do miss the aggressiveness of the original tune, when boost built more quickly.
So even though I prefer the Liberty GT in some ways over the 6MPS, each time I see a GT on the road I do still see them as the 'enemy'