Porsche and Mercedes: synonyms for luxury and performance.The high fuel costs in running luxury sedans have not frightened off the excusive clientele in the past. But things are changing. Cars can still cost a lot of money. But the upper market segment is beginning to think green as well. Porsche has introduced its Panamera S hybrid to cater for that market. Mercedes has brought out the S 250 CDI Blue Efficiency. Which is the more fuel efficient?