Indeed the pre-1995s are better in terms of quality and fit and finish. However, the post 1995s have simplified electrical systems and also from 1996, you could get stability control on them which makes it easy to drive the car all year round (this would only matter if you get snow and ice during your winter months). Do a search on W140s as there's more than plenty to read. Just remember that the W140 was a very expensive car, and probably the most overengineered car of all time. As a result, EVERYTHING is to watch out for when you purchase one. The electrical systems are the worst. They are very unreliable. Also, these cars have defective evaporators that will always need replacement around now given their current ages. This replacement is very labor intensive (about 40 hours) and so costs a lot of money. The key to purchasing a W140 is to research as much as possible on them so that you know what you're getting into.

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver