I've driven the ISF, GSF and both LCs, all in the space of an hour. On public roads I really don't think the driving experience differs vastly.
Yes, a day in each on some quiet Welsh roads may well be more telling, but even then, I doubt the difference between any of these cars would be that great.
I really did consider switching to an RCF, but apart from the updated interior and functionality, the RCF doesn't offer a big enough reason to switch. When I can't slap a warranty on the ISF, it may well be a different story.