Both good motors. As a cross country, day-to-day, A-B car I would take the valver every time. Handles better than a 5 IMHO and as long as you keep it up in the powerband you can make pretty rapid progress around the B-roads. You wont feel like youre thrashing it cruising at 110 on the motorway, itll be just another day at the office for the mighty valver! I would also say that a valver is rarer than a 5 turbo by a long long way.
If youre into quarter miling and cheapish mods/maintenance, get a 5, without a doubt. The other thing is that Id rather buy a 5 from an old geezer with a lot of history for 1200, than pay a dealer 1700 for a 16v. If valvers go wrong, or are at that stage of life where everythings about to go wrong, its gonna be big money. No dealer will properly service the car, theyll just paper over the cracks and charge you £500-£1000 more than youd buy the same car privately for.
All things being equal Id get the valver, if you sit in one and then the other, youll see that the Clio is a hell of a lot more of a car than a 5. But things are not equal in this case, I dont trust dealers and the 5 sounds like a genuine motor. Id have a good look over the GTT, if it looks standard and well looked after, offer 1100 and drive it away.