The Ka and the cinq are both very good cars, have driven both, and my mate has a Ka and i know exactly how they perform against each other side by side (!).
I know handling can be a personal preference, but the ka seems generally better, a very fun and involving car to drive.
The engines are both good, but different. My mates Ka is quite torquey, but thrashy at high revs, wheras the cinq engine thrives on high revs.
Pure straight line acceleration the cinq beats the Ka (at least the 60bhp version), and it is quite a good few car lengths by the time you get to 60.
I think both cars are brilliant because they capture the essence of a car that is small, fun to drive and neither needs to be expensive or powerful. they both look great too
6 to one and half a dozen to the other in my opinion!