There is not much difference in regards to the cross section size between a 140/80x 18 and the 120/100 x18.
There are size differences between brands however. You may find a 140 in 1 brand being wider than a 140 in another.
Regards the d606, they last well but don't grip well. The front is especially notorious as a bad gripper. I tried many many tires for both longevity and grip (in both dual sport and mx application) over 30+ years of riding. The rubber also works well on some bikes and not on others...sidewall flex, distortion over objects such as rocks, roots etc, it all affects the ride.
My choice would be a Kenda 777f as a road legal front for dual sporting...given you have the dry dusty gravel terrain I have round here. Lots of rocks as well.
That being said I am now using mx tires full time on the 690 as I have not intention of using it as a dual sport. My black top never extends more than about 5 miles.
I use desert tires on the rear. Ed608 is good and tough, so is the maxis desrt. Don't like the Mich desert...tough but sketchy. The m12 is pretty good in the brief winter, spring over here. 908 is tough and not bad grip but $$$ in Oz.
I think a big thing is too keep the tires pumped above what I'd do on an mx or enduro bike...20psi or so. The mass of the 690 tends to flex the rubber more on the rim.