Just to counter Rallye-Kings comments, Ive gone the other way (Cup to a 306 Rallye) and I prefer the sturdiness of the 306 over the Cup. It feels much more solid, even if its 5 years older.
I also diasgree with the comment that the Cup has more low down grunt. In standard form, the Cup only comes alive between 5000rpm and 7000rpm. There isnt much below there. With a cat-back exhaust, it does improve quite a bit. The Rallye benefits quite a lot from a decent air filter as the standard one is very restrictive. Two slight mods give each car a decent improvement.
If the 306 is a dated car, what the hell is an old Clio? I think they look much older than the 306 does. The Clio, to me, looks older than a Lancia Delta Integrale (and the Lancia is actually 5 years older than the Clio!)
If youre stuck on getting a Williams, then buy a 306 GTI-6 too and use it everyday. Save the Williams for weekend fun. You could buy both for the price of a Cup. If you decide on the Cup, make sure you get a good one. There will be a lot with dodgy paint, mechanicals and rattling. Choose carefully...