here's my 2 cents...
I've ridden quite a few different types of bikes in my time and have recently started riding dual sport full time. I owned a 2003 BMW F650GS, that I thoroughly enjoyed, but recently HAD to sell. Why?. . .you ask. . .because I test-rode a 2008 KLR 650 and fell in love.
What type of dual sport bike to buy? Well, if your pocket book can handle it, I'd suggest looking into one of the BMW 1100/1150/1200 GS's. These bikes will definitely perform off-road AND will also give you many miles of highway pleasure. On the other hand, if you'd prefer to stay with a smaller bike (self-admitted noob). . .or don't want to spend that much, then there's quite a bit to choose from in the 650 range. Just keep in mind, there's not a 650 dual sport bike out there that is really comfortable running 75mph all day.
Given that, it comes down to person taste, size, fit, etc. I really enjoyed my F650GS, but found the clunky transmission, low stature, expense of maint., and off-road handling to be. . .well, let's just say, less than desirable. Remember, don't LIKE what you ride. . . .LOVE what you ride! So now I've got about 500 miles on my new 2008 KLR and I'm loving it. Yes, just as expected, (once I finish the break-in) it's not too comfortable running highway speeds all day, but if one was to install a taller windshield and wasn't in too much of a hurry. . .she'd be just fine for some long highway miles. For me, the bottom line is. . .since all of the dual sport 650's are so comparable in performance. . .why not just get as much bike as you can for your dollar. . .and the Kawasaki KLR650 is a great bike that won't pinch the wallet too far.
Good luck in your quest. . .ride safe. . .and keep us posted.