I have owned a 1200GSA, F800, F650 Dakar, and rode the Sertao a few weeks ago at our local BMW shop, and it's a decent bike. Having seen a few of your other bikes (Didn't I catch a glimpse of a nice Duc 900 SS in one shot?) I would definitely suggest you ride a Sertao before you pull the trigger. It seems that the Sertao is just a tarted up version of the standard 650 with a little longer legs - not that it is a bad bike, but if you love to be a bit of a hooligan there just isn't a lot of snot there, and no really good way to get to it.
How soon are you looking to jump in? I may be moving my 990 ADV as soon as I make a run out to Colorado the end of June, and would make you a smoking deal on it - the hooligan factor is DEFINITELY there (Say wheelies in second and third while ripping up a gravel road) and it makes short work of long pulls of pavement.
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This, was dropped off by this yesterday, and I don't think the 990 will see much use this year after my trip to the mountains.
Either way good luck, and let me know what you end up with!