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?