The general trend is for most "New" bikes to be fitted with threadless headsets. But it doesn't matter what type you have on a bike as by buying a different stem- you will be able to adjust the Height or reach of the bars. Once you have got the bars to the right position- then I doubt that you would ever move them. My only thought is that Threadless stems are available at all bike shops and in a variety of lenghts and reach. The older Quill stems are not held in quite as many sizes as Threadless versions.

Saying that- For futureproofing- I would steer clear of the 1" stems as they are not the easiest to find at shops and for a bike that is ridden regularly- more maintenance is required on the smaller headset bearings.