The challenge to the SL500, Quattroporte argument is that they will cost a shedload in maintenance and lose at least 10% a year in value.
Whereas the 72k mile SL60 "shouldn't" cost too much to maintain, and will be worth twice as much in five years time - simply due to the rarity value of that "only 50 of these AMG models in the UK" claim. Look at how ordinary nice 500SL R107's are now asking the same kind of money now ( £40k.)
Great car - see the article comparing the SL500 and SL60 in the October Mercedes Enthusiast - but the value is all in the head of the owner. You won't drive it any faster than the SL500. - we drive to the limits of the road, not the limits of the car. And only the Geeks, like us, will know the difference. ("The Wife" will never understand the difference)
So from a usage point of view an SL500 or SL320 is the smarter, simpler choice. Cheaper to run, less depreciation, fewer worries about damage or loss. An SL60 is all about the "bigger than yours" thing, rarity and potential profit in the future.