As an owner of a 92 400e, I think the Q45 is a better machine than similar vintage MBZs.
Trust me, you'll see that the 560sec will have many electrical as well as mechanical problems crop up. The might not be major in a mehcanical sense but they will be in a $$$$$ sense!
I do think the 560sec is a fine car and he is right: It is one of the last great Mercedes. The next generation of MBZ, though more modern and better equiped, are made with much cheaper materials.
If I were to do it over again I'd would have bought another Q. Q has more creature comforts, much more responsive transmission, better/more durable cheaper to fix AC (2K repair for me that I'm putting off!), better ergonomics, better NHV.
The 560sec may handle better (my 400e certainly does) depending on setup.
Check out this forum if you're curious: Lots of good MB info here.
http://www.mercedesshop.com/shopforum/index.php3?