Pick up a copy of road and track, in this issue they did a head to head of the ST, GTI, and MS3.
Check out the forums, You've had a bad experience with your last car so you want to be well informed. The Gti forum has a lot of good info but also tends to have many biased fanboys. Ford ST nation is a forum dedicated to the ST and there's a sub section on any issues people may have already had with their new ST. Plenty of good info on there.
Lastly, test driving is going to tell you a lot, try to drive both of them on the same day.
Good luck, both are great cars!
SoCal Crew #7
DUB Edition 2012 Focus Hatch *SOLD*