I like threadless because it's a lot more stiff when I get out of the seat to climb or sprint from a traffic stop. Far too many threaded stems seem "noodley."

I have threaded forks and threadless adapters on two of my bikes. I like that system a lot. What's best is I can adjust the reach by flipping the stem and raising or lowering the adapter to the same height it was before.

Threadless stems are far easier swap out than threaded. Very few threaded stems have removable faceplates, making it necessary to remove the brake levers and bar tape in order to change them.

Finally, it's a lot easier to find threadless stems in an array of lengths and sizes. Threaded stems tend to be short and low. Finding one that's longer than 100mm and taller than two inches above the headset is difficult. And if you do find a larger and longer one, you may very well sacrifice stiffness.