Single or double treadle is a personal preference.
Some spinners like the single treadle because they can more easily position the wheel to their liking. With a double treadle, you need to be sitting square with the wheel.
My two main wheels (Kromski Sonata & Ashford Traveller) are both double treadle. Since they are castle style wheels with the orifice in the front, the double treadle is wonderful.
I have had a couple saxony style wheels that were single treadle. I haven't spun on a double treadle saxony, but think it would be awkward since the orifice is to the left of the treadles.