The SS is much faster and has a lot more power if you are talking about the 2008-2009 Turbocharged version.
I would take the GTI. Better reliability, better quality, more standard features, more practical, more comfortable, looks more expensive, has a higher resale value, better on gas.
Both cars require premium fuel since they are turbocharged.
The VW and Chevy placed 2nd and 3rd respectively in the $25000 sport compact comparison in the Jan 09 issue of Car and Driver (the Cobalt was only 4pts behind the VW on a scale of 1-250). However, the Cobalt SS was reviewed as funner to drive and a better performer. It made more power, accelerated faster and got better mileage. It also cost less.
The Cobalt SS additionally won the Lightening Lap Challenge in it's price bracket in the Nov 08 issue and actually out preformed most of the cars in the next highest bracket while coming just fractions of a second short of getting some time records.
Both were well liked. If you want performance, choose the Cobalt.
