I'm getting ready to replace my current 1983 380 SEL with a later, better maintained car. I love the W126's, so I'm looking at two very nice 1989 models.

I love my 380, but it was a foolish purchase, in that it was cheap. You know the rest. I keep pouring money in, but I still have long way to go to get it up to the level of either of the two cars I'm looking at. It finally dawned on me that it would cost me less to sell the 380 at a big loss and buy a better car, than to finish bringing the 380 up to snuff.

One is a 420 SEL, and the other a 300 SEL. Both have full records, excellent paint, beautiful interiors, fully functional accessories and about the same miles (185,000). They are also priced almost the same.
I drove them and they both drove very nice, with the 300 SEL being slightly slower. However, my real question is the expected longevity of the six vs. the V8, and the comparative maintenance costs of keeping one or the other in top shape.

Any advise on the comparative virtues of the two engines would be highly appreciated.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire