Reliability and cost
Reliability usually goes hand in hand with how the car is maintained. To be honest I have been on this forum for some 2 years and rarely do you get a post complaining about massive reliability issues.
The ML320 was purchased new and has a more than perfect maintenance history. Even extra fluid changes, flushes, etc., yet the center console switch system has been repaired once, replaced once, the MAS has gone bad twice, an Ignition coil got sick, the TCM is out at the moment, and more. A couple of support struts have gone bad, etc. It has to go in to diagnose and fix codes P0700/P0720, for the second time in 13 months. Big $ those!
My wife drives it like a baby. The worst part of all this is the incredible cost of parts and the relatively closed maintenance procedures.
As for the SL500, I bought it used and have little maintenance history but have done far more than specified. Some things that don't require service have been serviced by me, including, against the dealer recommendations, the torque converter and the differential. Every plastic part I touch breaks even though it lives in a garage. Parts so far cost almost as much as I would pay for a new car. My repair bill folder for the SL is 3/4" thick.
Topping this all off, MB used to be a client of mine, I sold them manufacturing and assembly equipment and have been to one of their factories in Germany. (I remember it was during the Apollo 13 problem). None of that reduces our costs.
Only problem is that we are now long retired, living on a very strict budget, so a new car is not in the cards.