forgot to mention the seat height is much lower on the gs. you must try the yam if your less than 5 11. I could manage it on road but couldn't get my feet down off road.
If it helps, own a 660 myself but ride a lot with a lad on a gs. We've both tackled the same stuff on the same day and both are more than capable bikes.
In my view...
GS is better on the road
660 is a bit lighter and after a good day on trails you start to notice, also I found it has better ground clearance.
GS I'm not sold in the canbus stuff, bits can be pricy
660 is simpler and easier to work on... Daft things like taking a sparkplug out
GS I found a bit soft but guess you could dial that out
660 looks better... In my view:)
As mentioned... The 660 is very tall. Try before you buy. If you lower it, you'll dial out some of the off road benefit. If you ride more on the road I think the GS will be better. If you do want full dual sport it'll come down to personal preference.