You could say this about a large volume of cars, it was the same when the williams came out - although admitadley it also had more power and better suspension but that age old debate of whether its worth an extra couple of k will always be asked.
I think somewhat it comes down to your own desires, i was always a fan of the williams and wanted one for years - so for me at the time it was worth the 'extra' it was one of those iconic cars i always wanted to say i owned.
If your keen on the trophy and want a ltd run to be a bit more special then go for it, if you are a little more blase i would go for a 17/82 FF/Cup dependant on what options you wanted and budget constaints. The only advice i would give is if the intention is to buy a car to track then it HAS to be the 172 cup for value for money - cant beat it.