Seeing as for a short time I had both Williams and 172 MK1 at the same time (and a 172 MK2 until Nov. last year), Id say Im fairly well qualified on this one.
Williams - simply stunning around corners. Never driven a car like it for handling prowess. Engine from a tractor, but I can forgive it that. Straight line speed not amazing, but able to keep up with the majority of modern hot hatches in a straight line.
172 MK2 - Nice interior, great toys, cool headlamps, wonderful engine, solid handling (when lowered - std. springs too mushy for my liking). Let down by being a wee bit too heavy due to those toys. Used to drive mine around with the rears out along with anything else easily removable. Made it more fun.
172 MK1 - Naff dashboard plastics aside, the interior is fine. I personally preferred the look and feel of the MK2 172 interior. My car is totally std. at the moment, so the handling feels soggy compared to the Williams. However, after inital disappointment at the handling, Ive realised that its just been set up softer than the Williams was from the factory. I can take my favourite peak district corners nearly as quick in the 172 mk1 as in the Williams. Its just fecking scary! The car leans to much, and because of the weight transfer on the soft springs it can snap out of corners that have been taken hard. Im going to lower it and fit k-techs camber bolts, and I think that will take it to somewhere closer to the Zen point of FWD handling that is the Williams.
The other bonus of the MK1 is that it is a useful few KG lighter than the mk2 172. Id be interested to see what the mk1 would be like with the MK2 gearbox in it though. If I ever kill the box, then Im going to have a mK2 box fitted in replacement.
As for the comments that people make about the old MK1 clio hot hatches getting more "looks" out on the road, it simply isnt true. Ive had all three varieties of hot clio now (okay, apart from the V6!), and you will get plenty of envious looks with any of the models.
In fact, more people have stopped to talk to me about the 172s that Ive had than the Williams! Shame that the Williams attracted more attention from the wrong sort of people
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In short, a Valver or a Williams can be tuned to be as quick or quicker than a standard 172. A 172 can be modified to handle as well or better than a Williams. Depends how much money youre willing to chuck at modding and insurance. I personally love sporty Clios, and having had loads of experience with them now, Id say that they all have a very similar character to them, despite 12 years seperating the oldest and newest models.
Sorry for the essay!