Mercedes used the straight 6 until 1997 in many different sizes: 2.6, 2.8, 3.0, and 3.2 liters with both single and dual overhead cams.
The V6 was used starting in the 1998 model year in the C, E, and S classes.

If you do a search you'll be able to read a lot of the opinions on the merits of both the inline and V6's.

Good Luck!

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".