ST200 for definite mate. My one was fine on the fuel, averaged 30mpg without too much hassle, on a run down the motorway would do 32-34mpg at a steady 80mph... sounded good, looked well (specially once polished up), was pretty quick in a straight line and got round the bends ok considering the size and (surprisingly heavy) weight.
I'd be really surprised if the Vectra GSi Hatchback has a bigger boot than the Mondeo...... the ST200 Hatchback has a MASSIVE boot?!
Cheers,
Grant