Of course, I'm biased...
Build quality and the roided looks of a 500E leave the 210 E55 far behind ...500Es are pretty easy to maintain/work on too (IMO). I think if you plan on keeping the car a 500E would be a better choice in the long run....for many reasons.
Both will put a smile on your face though when the pedal is floored....but overall performance/handling/fun factor has got to go to the 500E (not talking about straight line running here). No offenses to the 210 E55 owners but their cars are snatched off the regular assembly line and only a few bits go into making one as opposed to the whole package you get with the 500Es development and construction. 500Es were put together by the same techs that built the Porsche 959 on that cars assembly line. But, sure, the AMG techs over at Affalterbach are/were no slouches either...especially back then when the company was still sorta 'small'.
Hopefully a current 210 E55 owner will chime in on its attributes.