I can say that I was looking at a 600 last year before I bought my 500. Seems that repairs can really get you.
More importantly if you are looking at the 600 for the power, zero to 60 is only slightly better than the 500 due to the extra weight of the V12. Handling is also better in the 500 due to weight distribution imho.
More importantly don't make a quick decision on these cars.
It is hard not to do when you get the "fever", but that are lots of good 500 and 600's out there, if you know what you are looking for.
My 2 cents..

Matt

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1991 Mercedes 500SL
2001 Jaguar XJR
1967 Mustang