You've got some time to think this over which is nice.
The S10 is really a Japanese GS, so you getting the same basic capabilities as the GS, but with a Japanese flavor - water-cooled vs. air-cooled, vertical-twin vs. flat-twin, transverse crank vs. longitudinal crank, forks vs. telelever. They'll feel quite different, but they're both very similar in capability and comfort.
Don't let anyone tell you differently.... the GS is a great bike, has a very engaging character, and has a few incremental advantages: slightly more power (at the expense of slightly less mid-range torque), slightly better fuel economy, and slightly less wight. Although an acquired taste, I personally really like the telelever and the nice feeling of stability it gives the front end, especially on bumpy pavement and under braking. But I don't think it is as good a fork in the dirt. Also with the BMW, you get the option of the GSA with the much larger tank - this could be crucial if you want to really get off the beaten track. Given all this, I still strongly prefer the S10 mostly because of the outstanding Yamaha reliability history vs. somewhat spotty BMW reliability history and the much cheaper price, at least with USA pricing. (I don't think Canadian pricing of the S10 has been announced, so things may be different in your neck of the woods.) I also think the electronics package on the Yamaha is better sorted and overall just prefer the general look and packaging of the S10. YMMV.
The other bikes you're considering are not so similar to the S10.
The Guzzi has Italian character out the wazoo, but is almost an exotic by comparison, with minuscule dealer support and periods where parts go AWOL for months at a time. And Guzzi is a mfg that seems perpetually to lurch from one financial criss to another. Also, every comparo between the Stelvio and the GS has said the GS is functionally superior in almost every way. I love Guzzi's, but they're really a bike for a committed Guzzi aficionado willing to ante up the effort to owning something somewhat rare and exotic.
The KTM990 is an overgrown dirt bike and a high-strung thoroughbred compared to the BMW and Yamaha. This is an exciting ride. If you really want to take a big bike into the boonies, it is the obvious choice, but things like very-difficult-to-change tubed tires and the narrow seat work heavily against it in the touring role. It's also relatively maintenance intensive and while most KTM's are Okay on reliability, there are some real horror stories out there, so the risk of a lemon is much higher I think. This is a bike which really tips the scales towards the dirt - if that's your thing, this is the bike, but your pay for this capability in spades in the pavement touring role.
You might want to look at the new 800 Triumphs as well. Triumph is doing some VERY interesting things and I think the 800cc triple is going to be a sweet engine.
Good luck with your decision.
- Mark
only 125 miles away 
