Based on my own perception, the GTI felt like it pulled a little harder at low RPMs. The ST feels much more powerful in the mid range. While power is instantaneous in the GTI, the ST felt like it had a little (barely any) lag. I'm sure many people will disagree with me, but this is only my opinion, I have little experience with turbocharged gasoline engines.
For my money I'm probably getting the ST for these reasons:
1. Focus ST sounds better under throttle. It sounds addictively good.
2. I can get the Focus ST configured how I want for less than $30,000 out the door (leather, nav, hids). The only way I could consider buying a GTI is if I could get $4,000+ for my trade
3. Cost of maintenance. If you DIY the oil filter is $15, $45+ for oil in the GTI.