Its odd to hear that you are "torn" between the M, E, and SLK classses, because they are all completely different vehicles.
While price should be a factor, when you are shopping for a benz, the first consideration is what are you going to use the car for. Do you have kids? Do you like to go camping? how much space do you need? What kind of gas mileage are you looking for? You need to decide what you are looking for in a car, then start thinking about price, which will further refine your potential options.
Well I got a C coupe a few months ago and it's ok
... but to be honest I think I want something a little bigger.
I won't be re-leasing a C coupe, and I may not even wait until my lease is over to get a second vehicle.
I'm a single guy and I'm finishing up college so I don't work right now, and therefore don't drive much. I don't care about gas mileage, especially if I have two cars. I'm considering the E class because I have always been a fan of large elegant sedans. I'm considering the M class because I've never owned an SUV and think it would be cool, but I don't want a small SUV as I think it's cheap looking (same reason I don't want a small sedan) otherwise I would have gone with a GLK or C sedan. I'm considering the SLK because it's basically my current car with a sportier look and the ability to remove the roof.
The only reason I went with the C coupe was because it was a little cheaper than the SLK and being unemployed I didn't want a huge car payment. Now that I am about to graduate and will have a much larger income I am trying to decide on the next car I want to get once I find a job.
I really would be happy with the E, SLK, or M. I like all for various reasons, none based on utility, all based on different "coolness" factors. I decided I will pick the one with the best lease residual.