Other way around of course for me, but from my recollections of Williams ownership and driving over the same roads it would seem that the Willy is similar in driving character to the 172.
Theres the same low-rev torque and pull all the way to the red-line (lower in the Willy of course). I wouldnt say that the 172 is noticeably quicker in a straight line and the handling is very similar in terms of feel...or much closer than I thought it would be at least.
The major issue with Williams ownership is having the time to care for it. Its difficult enough to find a mint example - and its a crime not to keep one in tip-top shape. Whereas with a 172 you dont really have to worry about things like corrosion and age-related wear, it can be a bit of a battle to keep on top of it all with a lot of Mk1 Clios (well, at least if youre a perfectionist like me!). There are also the typical age-related Renault issues, like wheel bearings, rubbers, mounts, suspension etc.
If you buy a sorted Williams, then the niggles and repairs should be respectably low - but the "average" Williams or 16V could well need a tidy sum spent on it every so often...as I found out over 6 years of combined ownership of Mk1 Clios!
Id also suggest looking for a 16V if youre looking at a Williams. You could find a very tidy example for £2,000 - leaving plenty of dosh for maintenance and care.
You will lose out on many mod cons with either a 16V or Williams, but the driving sensation offered by both are terrific and the support youll get from the forum is a bonus.