Easiest way to compare/contrast:
W124 - "Fits like a glove" - Handles very well, Sportier
W126 - "King of the Road" - The ultimate highway car, Better ride quality, more room in the rear seat (if its a long wheel base, that is, otherwise its about the same)
Keep in mind, If you buy a 126, you have to be able to get used to driving a very large car. You better test drive one first.
One last note - price seems to be very similar, even though the 126's were considerably more when new.
2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino