Having had both of those cars and driving a 91 300TE currently, I'd base my decision on 1. Least rust and 2. If you drive it often, get the 300E it is just a more powerful, comfortable, better handling and probably safer car.
The 300E will have it's problems but so will the 240D. Just my opinion...good luck in picking what is best for you.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer