Like you, I think the W140 is an awesome car, with much presence. I have heard that most of the little niggling problems of the earlier years really were never solved. However, the early 2000 W220's also have MORE than their share of problems. I think in the long-run, both cars are going to be expensive to repair and maintain, but I think the fact that the W140 is more mechanical in nature than all the computer electronics of the W220, it will be easier to maintain. Not necessarily cheaper, but easier. Just MHO.
I DO like the W220 S600 with full leather interior though. It really dresses up the joint. :-)