Another way to look at it is the difference between a 1.4 and a 1.6 petrol theres a deffinite difference between them when driving and easy to tell the 1.6 is usually quicker driven the 1.4 petrol C4 and i know my 1.6 feels a fair bit quicker. Power wise there comparable the 2 HDI's but since i have not driven a C4 HDI i cant comment on it properly.