entirely personal preference
you'll have to make up your own mind.

Mine will be threadless because
- i like the look, or at least i don't really care
- i find them to be stiffer
- arguably, they're lighter (arguably, because your steerer will now be longer and heavier)
- easier to find a variety of stems with *removable faces*
- once I dial in my fit i never really change my bar height.