if you love your 420, you will be absolutely blown away by W220 ride comfort, power, handling and experience.
i use to love my S320 (W140 i believe) but once i got W220 it was a difference.
i still look forward driving it every day , even after 7 months now.
Sure, you love it, until it has to go in the shop every week and starts costing big bucks! Then you jump back into trusty timeless 420SEL and keep on going. I've driven many W220 (pre/post face lift) and I still have say that I like the W126's the best. W140 was close. The W221 I drove was impressive, but not timeless and not as reliable as the W126.