The quill stem has the advantage of "set it, lube it and forget it" on the bearings. Atleast it is on motorcycles.
Or, I am thinking one lock nut locked by the other locknut is better. The threadless has 1 much smaller nut/bolt, sitting much further away. But I guess in small light duty applications its the same. I infact like motorcycles that have a grease fitting in the steering neck and swingarm pivot too. That hasn't been on motorcycles since the 70's.