It is almost impossible to predict really. Certainly reports of CCCC vary wildly but don't expect anything too exciting and if you are lucky you will have a pleasant suprise.

If you only book a Citreon C1 your upgrade (if you get one) will not be very interesting. If you want the best upgrades you must at least book a BMW 3 series sized vehicle or equivalent, ideally an automatic. Then in my experience the agents are more open to offering really nice vehicles. The key in Europe is to book an automatic even if you book a very basic car as often availability of automatics is very restricted. Slightly off topic but a colleague of mine booked a compact automatic from Enterprise at Geneva and they were so short that they had to give her a Mercedes E Class as it was the smallest auto they had.