I don't have the most experience with these cars and can really only comment on differences between my 300D and my friends 240D. Both cars are extremely reliable. The 240 gets better mileage, like I think 4-6 miles per gallon better. However, his 240 is horribly slow. I don't think it's running as well as it should be, but I don't think my 300 is either. I can blow his doors clean off, mine's an auto and his is stick. I would never personally own a 240 as the 300 is quite slow enough for me.
I think if you lived in a very flat place, the 240 might be tolerable, but in SoCal, there are enough hills that at times, he's stuck in second gear because it bogs in third. In the same places, I pull strong in third but bog in 4th. Hope this helps. I'm sure others will have much more experience and info to add.
Frank.