I sold my Williams recently and am picking up a Fiat Coupe 20V Turbo tomorrow - so I think Im fairly qualified to comment on the two!
I originally looked into the 20VT last summer when Id sold my Valver, but the teenage dream of owning a Williams and prohibitive insurance put it on the back burner. In the meantime then, I owned probably one of the nicest examples of a Williams and it definitely lived up to all the hype.
If youve had a 16V and you loved it, then youll go crackers about a Williams. The 16V is 95% of the Williams and actually has more desirable equipement (leather, electrics etc), but this thinking doesnt tell you what the Willy is about. Trust me, from a drivers point of view down your favourite B-road the Williams is somewhere between 10% and 50% better than the 16V (already a legend in itself). BUT - theres not much point in getting a Williams if you wont benefit from this, because in mundane day-to-day use the experience is very close to a noisier, less comfortable 16V.
With my insurance coming up and the best time of year for selling upon us, I sold my Williams. I was sad to lose a fantastic example of a classic car, but Im glad to have owned one and am moving on now. The Williams is a great - but there are others I want to own too (e.g. Delta Integrale). It also happens that right now I need a car that can do bigger miles more comfortably.
Hence, I returned to the idea of a Fiat Coupe 20V Turbo. Its comfortable, stylish and IIRC its still the most rapid FWD factory car ever made - see this for a comparison with the E36 M3 Evolution
http://homepage.ntlworld.com/edperryman/Ccarticle1.JPGhttp://homepage.ntlworld.com/edperryman/Ccarticle1.JPG. The spec you get is incredible: four-pot Brembos, LSD, incredible pace, excellent equipment and even good build quality! Prices begin at £4,500 for a resasonable early car. A well maintained one will do spaceship miles without troubles, but a dog will bite you on the bum with very, very big bills. Thats what scared me off to start, but on doing the sums and spending time on
http://www.forum.fccuk.orgwww.forum.fccuk.org, the costs are about the same as with a Williams if you buy carefully. The driving experience obviously cant match a Williams down B-roads, but its surprisingly close and it has it licked in most other areas.
The 20VT insurance grouping varies between 17 and 20, depending who you listed to and whether its one of the special editions. My insurance works out at about £100 more a year over the Williams. Remember that IGs are mainly a guide for new cars (costs of replacing parts etc) and insurers revise their ratings individually as time goes on and patterns in claims become apparent. This explains the variation in quotes.
I pick mine up tomorrow and am happy to give more detailed comparisons if anyone wants to PM me. I bought my late 1998 "S" plater for £4,800 and have therefore walked away from the Williams with cash to spare!