One of the main reasons people go with single hungs is to get a more airtight window, as many brands of double hungs are very leaky. A single hung does away with one sash moving which theoretically will make a more airtight window.
With the Okna 500 double hung having a .02 air infiltration rate, that is completely a mute point (versus their single hung.) Plus, the difference in price between the Okna double hung and single hung is very minimal.
I'd go with their double hung every time given the choice between the two, ...unless of course the window was going into a moving vehicle or a boat.