Good question evowilly. Nothing about looks but all about dynamics, which is how to judge a car.
I can only comment on road use as I have no experience of the cars on track or rallying. There is no doubt in my mind that the 7 experience is more refined, for some to much so. Internally, the car is quieter and has less shake, rattle and roll ! I also think that the ergonomics of the 7 are better. As for performance, pure acceleration figures are a bit misleading. Both the 6 and 7 are about as fast point to point cars as you will ever find. For fast road use I would suspect that in theory, the 7 should make the better car. This is because the body shell is so much stiffer and although it is 40 kgs heavier, stiffness really counts. That is why Blade has noticed such an improvement in his 6 after fitting the roll cage.