I've driven the Mk6 GTi and the Leon FR but have the Cupra, I never got chance to drive the MK6 really hard as the salemans would probably have told me to slow down

I was looking for the GTi Mk5 before settling on the Cupra but I would say that the GTi is the better steer, its steering is a little sharper although the Cupra differs from the FR by havin alluminium in its suspension, I would say both the FR and CUpra could be made as good as the GTI.
If you prefer 3 door then the GTi is your only option however if you are happy with 5 doors then the FR well Cupra will win everytime, for the price its a complete bargin.