They have basically the same M112 engine, but the 2.6l in the C240 uses shorter strokes. The C240 gets slightly better MPG, but the C320 has more HP and came standard with a few more luxury features. Given the option, I'd probably choose the C320.
Supposedly, for some reason, the C320 is rated as 1 mpg better than the C240. When I bought new, the price difference couldn't be justified in my opinion. The same may be true in the used car market. As far as I know, reliability is the same.
Bigger is better I went with the c320 as I wanted more horsepower and torque. I would've kicked myself if after a couple years of ownership I wanted more horsepower! I'm 5 years running and no complaints. Best purchase and great compromise. Might not be an ///amg but gets me around town in style with decent hp and mpg.
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