Thanks for all the comments. You guys are the best!
Mark90M3: I really can't agree with you about interior quality. I have been unable to detect any difference in two X6 drives and two ML drives.
morty: Thanks for all your comments and for bumping that other thread. Just FYI: Running boards are not standard on the ML, just the ML63. And you can get active ventilated seats -- the ML550 I drove had them, perforated leather and all. Re 4-year maintenance: realistically that is worth $500 to $1,000; I just factor that into the price. Like you, I would like to be able to get leather dash (not even available on the X6 here in Canada) and HUD.
johnbmw6: The ML series certainly used to have crappy reliability, but according to both Consumers Reports and JDPower they are now ahead of the X5. Nothing on the X6 yet, of course.
Elevated: I agree about the ML63. It is insane. But I'm not sure I could live with one in the long term. Also, it is $$$.
xDrive501: I agree that X6 vs ML550 is slightly apples to oranges (well, more like Fuji apples to Gala apples since the X6 is derived so much from the X5). But I only have room for one plate of fruit, so it is still a valid comparison.
Deda : I was comparing the price of an ML550 with 21-inch wheels and the AMG body kit against a stock X6 50i with 20-inch wheels. Even factoring in lack of 'free' maintenance it is still an $8,000 difference -- which just rose to $11,000 as Mercedes Canada just announced a $3,000 price drop on the '09 ML550 if you pay cash.
Anyway, my heart is still saying "X6" while my brain is saying "Not so fast there, Buster..." I think I'll just chew on this for a week or two and let you know whether emotion or logic wins.
But thanks again for all the comments.