Hmm - obviously everyone gets different results. The facelifted '350 has a new engine in it, so I can understand changes there. The '55 has not been touched since it came out to the best of my knowledge, so not sure why it varies, or who listed the MY09 at 25?
MY09 FUEL ECONOMY (CITY /HW
300 : Manual: 17 mpg/26 mpg Automatic: 19 mpg/26 mpg
350 : 18 mpg/26 mpg
'55 : 14 mpg/22 mpg
MY08 FUEL ECONOMY (CITY /HW
280 : Manual: 17 mpg/25 mpg Automatic: 18 mpg/24 mpg
350 : 17 mpg/23 mpg
'55 : 14 mpg/20 mpg
MY07 FUEL ECONOMY (CITY /HW
280 : Manual: 20 (city)/27 (highway). Automatic: 20 (city)/27 (highway).
350 : Manual: 18 (city)/26 (highway). Automatic: 19 (city)/25 (highway).
'55 : Automatic: 13 (city)/21 (highway).
Unfortunately I don't have the 05 or 06 numbers handy..
Personally, I've managed to get as high as 27 on the highway (babying it, 55-65 ish), but I average 24 running "normally" (70+ average speeds over 600 mile trip. No tickets. :-) )
In town it varies - but 16-18 is pretty realistic. Stomping on it I can get it down to 14ish. I did the same type of driving in a 55 and managed to get down to 12 I think. Was a hell of a lot of fun, not sure my passenger agreed!
I know this is going to sound simplistic, but you need to drive the cars and see what makes you happy. The prices on SLK's are tanking, and you can get into a '55 for a really sweet price these days - and the 350 and 280/300 prices are free-falling as well.
Fuel efficiency really didn't play into my decision at all - I ended up in the 350 (as I've said 100's of time before) because it was right for me - right level of performance at the right ride quality.