Both locations are quite suitable. Perhaps the only real difference would be your commute to Irving from Prestonwood but you have the Beltway right there so you shouldn't have much in the way of delays.
The Prestonwood area is more densely populated than Valley Ranch. Either has easy access to downtown Dallas. I don't think you'd find a reason to not choose either one. Addison does have a lot of cool restaurants and a great July 4th fireworks display.
Everyone should have such a difficult choice.
