Ive owned both from new and have been delighted with both.
IMHO.......
The 172 is a formidable piece of kit all things considered, it has a superb spec for the price and provides a less compromised vehicle for all purposes. I sorely miss the Xenons and Aircon :-( but.........
The cup on the other hand is a different kettle of fish. Those tweaks on paper may not look much, but give it a try - youll be suprised.
The increased castor changes the atittude of the car, better turn-in and a more tactile link between the driver and the wheels. When coupled with the improved suspension and slightly wider track, I find the Cup a more accomplished drivers car.
From the word go the Cup felt more dynamic and lively. Since buying the Cup Ive removed almost everything that isnt required (rear seats, interior trim, stereo etc.. etc..) which has again improved almost every aspect.
The only point I would make is the Cup requires more forethought when driven hard. No ABS demands that you pay the utmost attention to road surface conditions; even in standard tune lighting the front wheels is easier than youd expect. Getting the most from your Cup takes more effort.
All things considered - youre going to be delighted either way