I have owned both and 400mi of slab on a 990 is no picnic either... The windscreen buffeting will scramble your brains. The 690 has clean air being there is a minimal windscreen. You will get wind pressure and you may get tired from that but the wind will be clean.
There are places in Colorado that may limit you on a 990 namely Poughkeepsie Gulch. Be prepared to fan the clutch a lot on the 990 if climbing the various passes. And ride with someone because picking up the 990 while at 13000ft will sap what ever energy you have.
You will also delight in the ease of maintenance of the 690 over the 990.
Having owned both, the 690 IS THE dual sport bike biased to the dirt but that's what I ride.
I can explain it to you but I cannot understand it to you
2013 Guzzi Stelvio
2010 KTM 690 Enduro
2009 KTM 400XCW Plated