If the massive power isn't necessary for you, and you still like the compactness/mobility of a C-class... then you likely answered yourself - get the C350. Many, including moi, vote for the C55 - it is surely a monster once you drive it. But then again if you can sacrifice all that wrath and fury, then go for the C350... after all, at least the C350 still has ample power with its 0-60mph dash in 6.1 secs.
I had a 2006 C350 Sport spoken for before it got off the boat in Jacksonville. Black on black, the only options were the sunroof package and Sirius. That car has since gone on to Mercedes heaven - but it gave its life for mine and proved that it has enough power to get you into trouble. Haha.
It's faster than the factory figures. Even with the 7-Gtronic, C/D or R/T tested it at 5.6 to 60mph - a figure I was able to duplicate routinely in its all-too-short 12,000 mile life with me.
It may not be the all-out barn-burner that I'm sure the '55 is (I've never had the pleasure), but as I said it was plenty fast. On a long road trip right after it was broken-in, I marveled at the car's ability to hum along... with the cruise control set just below the electronic limiter at 130mph (they say they're regulated at 133, but mine once ran to 135).
For passing, it'll also make a kickdown dash from 65mph to about 90mph in the length of an 18-wheeler holding that 65mph pace.
After I'd already signed for the car, R/T did a tiny little blurb (sometime between April and June 2006) pointing out that, for '06, the 3.5L mill from the new 2005 SLK was making its way into the C-class. For the life of the car, and as I've continued to look back fondly ever since, I agree with their assessment of the 2006-07 C350 Sport as an enthusiast's "perfect daily driver".
It's no AMG, but don't sell that 350 short. It won't have the prestige, but if you put a nice set of Michelins on her and let her off the leash as often as possible, it's a tremendously rewarding car.
As a conterpoint, the '55 you mention is a 2005.
I'm all for ANY sport-classified C-class from the 2005 model year forward on account of the design of the center stack alone. The better radio and steering wheel from the SLK doesn't hurt, either. My sister has a 2004 CLK and that old-look interior, with the casette-tape head unit and vacant bins, in my mind just ruins an otherwise great car. The instrument cluster on 2004 and prior C classes also makes me frown.
Also to consider: even if you don't need the power, or the status, the '55 AMG will satisfy in terms of sounding and looking better.
For all the fun it provided, the 3.5L V-6 didn't sing like a BMW 6, nor did it roar like only a Mercedes V-8 can. It's actually somewhat quiet. I you're like me, and basically get off on the idea of the 350's "stealth" quotient, that's great. I'm not saying the 350 leaves one wanting for a sense of involvement, but having that 5.5L in the car's belly must redefine involvement.
There's also the issue that the 350 sport will be visually indistinguishable from a 230 sport on the outside (almost the inside, too - for 2006 the interiors were the same, save for the fact that the 350 has dual power seats standard; for 2007, I beleive the diference is that the 350 went to the black woodgrain trim while the 230 kept the aluminum - at any rate, I consider a departure from the aluminum a drawback). Again, if you like that, it's great. I left a whole lot of surprised Jap-tuner kids in the dust because they thought they were picking a fight with a 230. But you do end up having a car that was significantly more expensive, while not appearing so.
The AMG will also have those wonderful wheels, instead of - again - the standard fare carried over to the 350 from the 230.
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