Let us just all assume these phones are equally strong, rugged and reliable; so the only questions becomes how small do you want it in your pocket and how big do you want it folded out.
I want it to fold out form a 6" smartphone screen to a bigger screen maybe 3:4 ratio. The typical 6" tall by 3" wide smartphone is plenty portable for me, and I think the current smartphone screen is fine for a quick check by I COULD use a bigger "tablets" sized screen for looking at maps, movies, images, etc. I square while folded "billfold phone" that turns into a 5.9" x 2.9" screen does not offer much that I can use in terms of reduced size. JMO
In everyday use I think there are times when more screen space so images, movies, websites would be easier to look at or use bigger. Moreover, I think my pocket would easily accommodate a smartphone that folded out, my pockets are not hurting for room or accommodate the height of a current smartphone.
I really want to know what people think. Folding phones are coming, but I think they are actuality going to be a form factor that is just standard in five years. Before iPhone showed up so many phones were folding phones, or phones with a slide that revealed a keyboard. I think folding phones inevitably the next step once again. They might be niche for next two years, but they WILL take over eventually. Thus what is the form factor you think you would want? Like I said, put aside your ideas of if one type is stronger than another...if it was just 'folded size' and 'folded out size' what form factor do you thinking you would use.