but what about engine quality, rusting of the bodywork, suspension quality, security (of me and the passengers), service cost
compared of these two models?
Both chassis have faithful followings... I may have a w210 now but I have (and still am) been a big fan of w124 cars, especially the coupes.
Here's my outline on the subject:
engine quality - for the most part both have nearly bulletproof powerplants in petrol and diesel versions, but continuous maintenance to the T is a must for longevity
rusting of bodywork - this is where the w210 fails, which is why a PPI for corrosion is a must! it's hit or miss - a good number were affected by quality control issues during production while many were thankfully spared, but since the w124s are older then rust can also be susceptible
suspension quality - coil springs and struts, McPherson on the 124s, front double wishbone on 210s, and 5-arm multilinks on the rears... both have good components for smooth comfy rides with a tad of firmness for a hint of sporty handling, the 124s are more tossable though than the w210s (non-AMGs) since they're lighter and nimbler, but if you want the ultimate in power & handling for a MB - then an AMG is the way to go
security - both chassis are vault-like (but what MB really isn't); w210s have a slew of advanced safety features like ESP, BAS, ABS, ASR, front & side & curtain airbags galore, etc.... w124s are still very safe vehicles for its era with ABS and SRS, oh btw both have 4-matics as options
service cost - obviously a higher maintenance cost for the w210s, w124s are more user (DIY) friendly, but w210s routine maintenance are still fairly easy to service on your own
Hope this helps... I'm sure many others have good suggestions so just keep on doing your homework until you decide.