Well, my Cup arrives tomorrow so I aint quite an owner (yet), but here are my reasons for choosing a Cup over a 172 Sport...
Colour
Alloys
Peformance & handling
Price
Better Spoiler & front splitter
The blue coloured interior detailing
Exclusivity
If they did a new Clio with the luxury spec of a 172 Sport, but had the rest of the Cup stuff (colour, alloys, spoiler, handling tweaks, etc...) then Id probably have gone for that!
Price wasnt the main issue here, as I got my car cheap anyway and could get a full UK 172 from a main dealer for less than a Cup costs list price!