There is nothing more to an R230 than the SBC and Vario roof system over a W220 so by definition they cannot be much worse, SBC and the roof are both very reliable too. The rest of the car is mechanically almost identical, the same ABC system, the same electronics, the same engines, etc etc.
The fact that my SL has actually been better than my S is pot luck, but they cannot be worse than the W220 by the fact that they are the same!

The only thing you dont want to do with either car is park them outside, my SL has almost never ever spent a night outside getting wet, it sits in a nice dry garage and has done from brand new. This may account for some of the increase in reliability which i have seen come to think of it. Reason being that they both suffer from water ingress, the SL especially so with its millions of roof seals.