I am very familiar with both cars. The 420 is not worlds apart from the 380 in terms of power - they are similar regardless of what anyone else will tell you here. They are not rockets but are adequately powered and really come into their element on the highway. Only the 560 is much much faster, and I speak from personal experience.

My 85 380SE is in excellent mechanical shape and gives me 16mpg around town with a very heavy foot and the A/C on. On the highway under less heavy conditions and with the cruise on it gives just under 20mpg.

The long wheelbase models have unparalleled rear seat legroom, but the tradeoff is a vehicle that can be difficult to park and maneuver. Sort of like driving a cargo van, if you have ever done that before - takes lots of planning before making turns. The shorter wheelbase models seem a bit more nimble both in traffic and in the parking lot.

Either way, you can't lose with a W126.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280