Have owned them both. they are basically the same car EXCEPT the power aspect. I doubt you would be happy with a 240 after driving a 300. They are very slow, but are they reliable. The mileage really is that much different, depends a lot on the engines stage of life. well just my opinion

__________________
Ricali

03 C240 4matic wagon
95 300E 234,000
7 prior 240;s 5 still going
81 300sd gone
65 230sl gone
49 Studebaker Champion
90BMW convert.167,000
60 Dodge D-100