Great taste in cars! I just had a similar experience, let's just say "the might as well's" did me in
BMW was taking back my 550 for reliability issues and I was contemplating what to get. I drove a 650 vert and that conversation was over! I would seriously look at an M sport 650-especially the 13's with 445 hp. They sound and perform badass. I went with a loaded 650 and was totally happy for a month or two until the cold weather came. Then my wife said something so stupid, I mean brilliant, and said " you really should get a truck to drive in the winter". I literally had the speed dial for the dealer going before she finished the sentence. She said what are you doing, when I said calling the dealer- she just laughed.
Originally I was going to lease a fairly loaded leftover X5 diesel, $71k sticker, for $589 a month with about $2k out of pocket. Then I accidentally asked him if they had any ridiculous deals on a 750, to which he replied "yes". In the end I got a 750iL X drive nicely optioned for the same price. My wife said "but I thought you were getting a truck" to which I simply explained it would be an insult to not get a 750 for the same price lol.
Now I am left with a desire to do something with her ESV. Too bad BMW doesn't make a full size truck, if the X5 was a little bigger I would do it. I think I might look at the new GL. We shall see.
I really do have a problem.
Good luck with whatever you decide, but I would go for a 6 over an SL- I just don't like the look of the new car....