Since you're locked into an automatic transmission, I think that the 320 is the clear choice. The only reason I got the C240 was my pent up desire for a new manual transmission Benz sedan(not to mention the fact that every car at my place was broken at the time and walking didn't cut it). If they put the six speed in the 320 and I am able, I will replace my 240 with a 320.

To me it's kind of a shame to have to buy one of these cars with an automatic. But I realize that; a) I'm in the extreme minority as far as transmission preference goes and b) this is my own personal opinion.

Good luck and enjoy it whatever it is,