I've got a 98 ML (knock on wood) it's reliable and the bugs have been worked out. Mileage around town is between 19 and 21 mpg.
Highway driving is fractionally better. If you're decideing to get another 6cyl truck then you're splitting hairs. I believe the mileage on the 6cyl MLs is pretty good for it's class. If you're concerned about paying more for gas, and want a truck, maybe you should consider getting a Japanese 4cyl truck instead.
It won't break down, and you'll get good milage.
Just a suggestion.