I suspect build quality would be lower as would residuals
I doubt it. I've driven many Fiestas and owned a mk7 Zetec S from new and can honestly say it is the best car I've driven (in terms of what it is). Build was superb, perfect reliability, brilliant handling and for sheer fun, although it was underpowered at only 120PS. Best part was the cost. I purchased for £12.7k and sold for £6k after 4 years! Car never missed a beat and never needed a drop of oil.
So the only problem with this car was the power and the new Fiesta ST fixes that. They may have made the ride too stiff though... Its not even that small. Its compact, so parking is a breeze, but inside the Focus offers only small gains in space. Features are obviously less than on a GTI, but on a GTI you are paying for it. On my Zetec S I added almost ever option you could and still paid less than £13k (admittedly I got an unbelievable discount + £2k scrappage scheme so the car was RRP at £17k).
In my opinion this car is a serious contender and anyone who says otherwise is engaging in badge snobbery. It offers a huge amount and at vastly less moulah than a GTI a BMW or any other of the traditional hot hatches out there. The only faults you can really level at it are that its less featured and can't do 0-60 in 6 seconds, but these are things you can't complain about when you see the price tag. If you have to have the best quality and performance then buy a
BMW GTI (my bad, slipped for a second there

), if you want value for your money then Fiesta all the way.
