I don't know much about threadless headsets/stems, so just asking some questions here. Did the LBS say why a threadless setup would make it easier to change in the future?

I'm accustomed to threaded headsets and stems and they're pretty easy to change. The only pain in the azz is undoing all the stuff on the handlebar, which is not a function of whether the headset is threaded or threadless, but whether you have the kind of stem that opens up at the extension so you can just place the bar in there and not have to remove everything from one side of the bar so you can thread it through.

Again, these are just some things to consider.