I don't know... having a DRZ myself (although not a supermoto), I tend to disagree with some of the comments.
You can get a better seat for a DRZ. I have a seat concepts seat, but other options exist such as Sargent or Corbin. I've done trips up to 2000km on my DRZ and I didn't find it too bad in the way of comfort. I wouldn't expect to do that many miles on an interstate type highway... but who wants to ride that anyway? To me, a SS doesn't look comfortable at all -- I prefer an upright seating position. It is also very easy to change where you are sitting on a dirt-bike... just slide up or down the seat until you are comfortable again.
For mileage... I've rarely gotten worse than 4.5l/100k and sometimes as good as 3.5l/100k with the stock 400s sprocket sizes. That seems pretty damn good to me... better than my Vstrom usually gets. Sure my range is only 150km, but aftermarket tanks exist that can double that range.
Finally, my insurance on the DRZ is $240/year for liability and comprehensive. Yes, I am an old guy (okay, actually middle aged)... but I doubt I could get on an SS for much less than $1000/year.