My 7.3L Powerstroke has 100k on the clock and is showing no signs of slowing down whatsoever....it has actually had no major engine work done AT ALL, just regular oil and filter changes. My most significant issue has been flat tires, I've had a few experiences...once driving to Phoenix in August a rear tire blew, I changed it in the 110F+ heat, only to have the spare shred itself a few miles later!
Had to have it towed and have a non-matching tire put on (the shop in the little town didn't carry the Bridgestones I was using).
Other than that, it's an extremely reliable truck, it starts immediately in freezing Tahoe temperatures (no glowplug work has been done), it's been as far east as Yellowstone and as far south as Cabo San Lucas. The motor is definately more reliable than the Benz, even if the car itself isn't (Fords have little quality niggles)....but comfort aside, I would highly recommend it.
I've never driven the 6.0L but have heard good things about them...seems like they've had some problems though. They seem to accelerate quicker than the 7.3 (might have a better trans) but the 7.3 is definately bombproof....and a lot cheaper to have fixed than the Mercedes!
7.3L has a wastegated, non-VGT turbo.