Keep the Accord, thrash the 300D, then get the 300SD. Like Hatterasguy said, the Accord will never let you down like the two Benzes will sometime. By just being newer, the Accord is bound to give less issues that the two Benzes. And when issues do occur, you can get any cheap backyard mechanic to fix the Accord for you. Also, I couldn't see your location in your profile, but if you live in a place that gets some ice and snow in winter, then the Accord, with its front wheel drive will be a good car for those bad road days.

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver