The Dua carries in pocket much better than the SNG. (the severity of this difference changes based on if you get a lego, cc, or dgg SNG)
The SNG offers more grip options for sure. The handle of the Dua MIGHT be too short than some people with big meat paw hands would like. I have medium sized hands and love the handle of the Dua.
The SNG gets the nod with variety choices -- the Dua is only s30v, which is fine, but some people want to experiment with some of the newer premium steels like Z-Wear, PD1, etc. Also SNG obviously comes in DGG, CC, Lego and a variety of finishes/colors.
The SNG is more readily available. Warranty is a toss up but we all know Strider's is an excellent warranty for folders. Never used Grayman warranty but I suspect they are very accommodating based on their customer service.
That all being said, the cheapest SNG from a dealer is $400...you can get a Ti/Ti Dua for $275 from Grayman. If it was ME, I pick the Dua every time. Just trying to lay out a few points for you. Best of luck!
PS: both knives hold their value exceptionally well, especially if you don't use them much. If you have the financial means, I would get both and then decide which you want to sell off. You might take a hit of $20-$30 but that's it.