2003 S55 vs 2003 S600???
I would seriously suggest 04+ from the interior electronics standpoint. I love my car so far, it was a big jump from the W140 228hp barn that I drove previously, which itself was the finest automobile I'd yet owned.
'03 and older cars have the now obsolete D2B fiber-optic network for audio/telematics in the car. As I understand it, MB was one of the industry pioneers with using fiber in cars, but by '04, an industry standard system (MOST) had been developed, and is in use by more than just MB.
What that means to the average owner is better upgradability, and availability of factory features and aftermarket add-ons. (I can't even get integrated Sirius/XM on my car...). Most S600's are gonna come stocked to the gills with options, but if something is missing and you want that feature, there are more and cheaper options for the MOST-based systems than for D2B-based systems.
As a quick example, I wanted to add iPod integration to my car, which has the Command audio system. It cost almost $600 for the adapter box (the Gateway 500). What that got me was a "CD Impersonator". I've had to disconnect my CD changer which is no big deal, as it's in the trunk, and I haven't listened to a CD in a car more than 5 times in the last 5 years. Now what I have is wonderfully clear iPod audio, but all I see on the Command display, and in the instrument cluster is "Track 5", with a timer that starts at 10 seconds and increases to 59 seconds, and starts over at 10 seconds. There is no way to display iPod text on a D2B-based Command system.... Truth be told I primarily use my iPod to listen to audio books, so it literally plays for days and days without my needing to change anything, but if I wanted it for music, I'd be stuck using the iPod to browse through the music, instead of the ideal situation of using the head unit or the steering wheel controls. As for Sirius, which I had on the previous MB via an aftermarket radio, my only options are to get a portable Sirius tuner and connect it to the "CD Impersonator" (which thankfully has RCA-in jacks), and I'll still have to use the Sirius tuner to change stations, etc.
For me, far from ideal, but better than nothing at all. I got a screamin' deal on my S55, but I spend a lot of time in the car, and occasionally I wish I'd waited a bit and found an '04+ S55.
If $ are a concern, wait 6 months and the '04's will be where the '03's are now. If we get another spike on oil prices, the S600's may be cheaper than the S55's... However, having said that, if $ are a concern, possibly an MB isn't a good choice to begin with. "There's nothing more expensive than a cheap Mercedes."
Dimensions. The factory HE is about 241.5 cubic inches; the one I'm working on should have at least 520 up to 700, depending on what I actually get packaged. Packaging is dependent on getting the factory oil cooler moved forward about an inch. Same idea as the difference between a smaller 1 core brass/steel rad and a large 2 core aluminum rad.
although the s55 has a rumble like an american musclecar, an s-class is more like what an earlier poster said, a bullet train, delivering a quiet surge of accleration with little effort! AMG package or not the 600 is the ultimate sleeper and will always turn heads for decades...
oh, also there is more options for aftermarket tuning for high hp gains than a 55K. I still love the s55..but that v12 biturbo is no joke.
only downfall is the cost of maintenance for cars of this caliber.
S600 is ultimately a better buy but get what you're looking for. if you want the "feel" of chest-pounding torque, get the 55K, if you like quiet, unfathomable power, get the 600...

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