congrats. my reply to you is my first post! i owned an A4 prior to picking up my 300 sport. compared to the A4, the 300 is much heavier and slower. i did not test drive the 350, but from what i understand, there is a noticeable difference between the two. it really depends on what you are looking for in the car. if you want something truly "sporty", i reckon you should get the 350if your wallet can stand the hit.
for what it is worth, some folks think the 300 is quick.
It is quick. A very nicely engineered car.
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2008 Mercedes C300 Sport mars red/black
2007 Lexus IS350 Sport granite metallic/gray
2004 Porsche 986S seal gray metallic/black/black
2002 Jeep KJ Sport white/gray
having driven the C300 & C350 back to back on test drives, the C350 does have a little more "bite" in the acceleration and in the turns. My wife and I could definitely tell the difference between the two. Is it enough to warrant the price difference? That's something for you to decide. if you haven't already, go to the dealer and test drive each one back to back, you'll get a feel for what I'm talking about.
The C350 is not blinding fast compared to the C300, I mean its not like driving a C300 and then driving a AMG63, but there is enough difference that the car feels a "tad" faster".
It's not just the power, it's the refinement with which the power you summon is delivered. My thoughts on the two cars can be found on this thread in another forum: