The plastic in these phones are not your average plastic. This plastic is made to withstand to a degree - flex, shock, and in some cases scratching.
If you drop 2 phones of equal weight from the same height, one being plastic and the other metal... If both land on a corner, the metal phone is more likely to crack the screen.
The reason is because the plastic can absorb some of the shock, while the metal transfers most of the shock to the glass.
With all that said, I have no preference in material as long as I like the phone.