Owning both a w124 and a w126 I would suggest that either car will work, and both have their charms. Both seem equally reliable, and my maintenance costs to keep them in tip-top shape have averaged around $1000/year. (both cars were in good condition when purchased).
The w126 is a joy to drive - its big, looks great (I have a coupe) and is wonderful for a long trip. My w124 is a diesel, nimble, good road feel, not as big (not much bigger than a Camry, but more functional IMHO). I plan to keep both for a long time.
I suggest that you buy the car in the best shape - with a history that you can verify. My w126 was originally owned by my mechanic and was maintained by him for several years after I sold it. He was clear when I bought it was "the only thing this car needs is new rear accumulators and the passenger side seat belt extender doesn't work" and he was dead-right. I have repaired both and now have a car that should run for a long time. History counts!

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John Gillespie
1988 560sec - 192k miles
2006 CLK500 - 40K miles
1995 E300 - 202k miles