I have only borrowed a C-class and I liked it (especially the Xenon headlights). It was a loaner vehicle and just after I bought my ML, so what I really wanted was my ML back.
As for my experience with the ML. I absolutely love it. I drove from North Carolina to Boston one weekend and after arriving Boston at 9pm on Saturday night, I got out of bed at 2am and drove to Montreal, Canada (nearly 800 miles in a weekend!). That was a crazy weekend, but the thing that amazed me was that I never got tired through the whole trip. The ML is, in my opinion, an amazing vehicle for long trips.
I have never driven a 320 or 350 (or ML55, for that matter), but I really love the power of the 500. The truck is made to cruise effortlessly at more than 90 mph (which you can do on 128, 93 and the Mass Pike in Boston... Not so much in North Carolina
). Even at 90mph, the truck can accelerate like mad. It has a lot of torque! I can't compare the feeling I have with my ML to any car I've every driven. There are not many cars that can catch it pulling off at a stop light. With that said, I don't recommend any hard acceleration anywhere without checking for unmarked cars first.
If you've never owned a car with real power, this will take some getting used to.
The other thing I love is that I can work on the ML easily. So far, I have replaced my brake pads (and will do the rotors soon), spark plugs, changed my oil (really easy... I wasn't even there for the most of it! That requires explanation, but later), and some minor things. I think it's well designed and a pleasure to DIY. Very much a money saver!
The down side is the gas mileage. Now that fuel prices are so high, I'm spending a lot on gas. I fill up on Mondays and Fridays and get something less than 300 miles to the tank. On the highway, I get much better, but in town it sucks. Of course, the 320 and 350 will be a little better. Because of the high price of gas, I don't race off at lights so much now, but I like to think that it's better to have and not use than to not have at all!
I found mine on Autotrader. We are lucky in Boston that there are so many MLs on the market... you really have your choice. In North Carolina, not only was I the only ML, but one of 2 Mercedes in my small home town. I bet I could get a mint for it there.
Oops! I digressed. Take advantage of your position and don't look for a best place to buy. Look at every ML and determine the best deal for you. I recommend buying with less than 60k miles (mine at 59k) and purchasing a warranty. I bought AAA for ~$3200. A month later, the warranty paid $2000+ to fix my air conditioning flap (mostly labor... they had to remove the whole dash to reach the 10 cent part.) With that said, happy hunting!