Someone told you wrong: The 1.4D-4D diesel has very similar performance to the 1.4VVTi petrol. The diesel is a second or so slower to 60, but it produces far more torque, and makes it's power lower in the rev range, so has far more overtaking power without the need for changing down, and far more flexibility in traffic. It will also average 16mpg more than the petrol. The same applies for most cars, to the extent that in the British touring car championships, Seat diesels (which are restricted to the same size as everyone else's petrol engines) are easily on par with their competitors, and have a huge torque advantage.