Single seals are a recipe for disaster for airtightness, i think some of the flush ones have 2 seals, but they are the same type, i.e. friction fit, so will never provide a great seal, and will be susceptible to ripping.
As @Nickfromwales said, you only want to be doing windows once, they're an expensive item, so the important things are seals, frame quality (in terms of insulating factor, the number of chambers etc), and the thickness of glazing unit you can fit.