Both cars are priced equally for what the year and model. I'd choose based on condition: the one that had no paint work (if there is one), and the quality of service records. If they were both the same in those respects, I'd probably go for the 91 because they're the most refined (New seats, speaker system, tougher engine components, 5 less years of age) - the 86 may have a few more bugs in it.

Documentation of the miles is also important. Both of those cars mileages' translate to less than 10k per year, which is below average.

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2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey

SOLD:
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino