I've got a S5 cab that's 18 months old, and I would not get the diesel, with the roof off it just wouldn't have the appeal, half of the fun is the acoustics of booting it and that's really not going to work with a oil burner, that said you do see a few diesel soft tops but i've never got it.
I haven't had an issue with paint chips, and in 18 months its only really had one issue with the start stop system, unfortunately MK Audi took 3 goes to fix that which left a bad taste.
I did curb my 2 nearside alloys last week when some ****nut came round a bend in the middle of the road, bought a dashcam now (after the horse has bolted and all that) but i'm not looking forward to the quote to clean up the alloys.
The S to RS jump is massive cost wise for what you get in performance, but the alternative is getting a 18 month old car, im going to keep this one for another 18 months at least, and I will see how that market is when i change, life changes could make a Cab totally impractical as well, children dont fit in Cab with all the associated crap you have to move around.