Here's my quick and dirty opinion:

W124 (like your E420) = nimble, sporty, more performance oriented, more modern styling appeal
W126 = more comfortable (especiallyl for passengers), but less sporty, not as flat in the turns, harder to park, much much cheaper to maintain, very very easy to work on, styling beginning to look dated

I have driven both types for years so I speak from experience.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280