A few years ago I had two 83s, a 300SD and my 300D.
All things being equal, I'd rather have the 300SD.
In the end, the condition and pedigree of the 300D trumped the SD....

The SD will have power seats, and a bit more room in the back. Width is virtually a wash.

I didn't see any significant mileage difference. Car condition, driver's attitude, and terrain are more significant. Both cars while under my watch delivered similar mileage.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles