I've just done 10 months in my old Shadow blue with 18's (which had 80K on it when I bought it) and yesterday I bought my new one with 17's, (again with 80 odd K on it) and, whilst I need more time to make a proper judgement, I have to say the whole driving experience so far is so much better on 17's.
The car in general feels in far better health for having been on 17's it's whole life. There are no rattles or creaks whatsover, whereas my old car had a few of these. The handling seems excellent, and the ride is in a different league to the old car. It feels very comfortable at high speed, really well composed, and I felt far more confident. It's a lot smoother, yet still grips really well, and the communication and feel is brilliant, but then it was on 18's as well TBH. I was going to sell these on straight away, but I've been so impressed I'm tempted to keep them for a bit, at least until I've driven the car on 18's for a bit.
But.........
It does look like a Dinky toy! I'm not sure I can live with the looks. The arches look quite empty, and it's crying out for 18's. It's a very difficult decision. The sensible choice would be to keep them and maximise the driving feel, but I'm generally not very sensible!