How about both? Meaning a foldable phone that has a display on the outside as well. Want to use it one-handed like most current phones? Just keep it folded and use the 'outer' display. Need more screen real estate and be willing to use both hands, unfold it and you get a small tablet.
Of course this idea doesn't make an already relatively thick phone thinner (to the contrary), but we might get there sooner or later with an acceptable thickness (though the minimal thickness of the unfolded phone will at some point be limited by mechanical stability concerns and this might result in a thickness of the folded phone that is too thick for most people).
Folded one will be substantially thicker and heavier.