There is a 19 year difference between them, if resale is a concern in the next couple of years the CDI will likely be a better return on resale. The W126 on the otherhand is somewhat of a classic, values have bottomed so from here on out well kept models should increase over the long haul.

I'm partial to the better build quality of the earlier cars, and a big fan of the OM603, but that's me. As Chris mention, buy what gives you the most satisfaction for the $$

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)