Considering 2008 ML350 - Any Advice

Hi there,
I own 2002 C240 and love it. The other car is 2005 Chevy Malibu. Its time for me to replace Chevy Malibu (troubles with so many things) and
I am considering 2008 ML350. Yesterday I went to dealer and looked at few. I liked the interior and the overall design.
I drive 90KM/Day mostly highway. Should I consider ML350 gas model or ML320 Diesel CDI bluetec? According to sales person, service is due for gas model every 20K KM and for diesel its every 15K. On every service they add some blue liquid for diesel models that runs around $75.
I am looking forward to expert advice.
Thank you.
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