Yes, I think that an inline 6 sounds more refined. I drove the CLK 320 today and it sounds more gruntled, like meatier, or more of a low tone. The C280 and the CLK320 seem to be about the same speed, acceleration-wise. But the inline definitly seems smoother and more refined.
My mom's Porsche has a flat six. I never really thought about it though. How are they able to get more power out of a 3.6 liter flat? That engine is just loud!

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)