I would have a ST24 or ST200 over a Vectra GSI anyday.
I had a GSI estate for 6 months and it was rubbish. For ever throwing up fault codes and the rear bushes, which are a very common fault, are outragously expensive to buy, let along get fitted. I had an ST24 for a year, only just sold it, and it was as good as gold, just put petrol in it and topped up the screen wash.
The GSI way be a little quicker but lets be honest none of these cars are really fast are they....
2p