Lots of factors. The seat is bigger, the cabin is better on most of the VS fleet. The mini-perks over economy might tick a box.
However, there are still poor seats in PE and weaker Economy to Premium differentials across the fleet (I am not sold on VS A350 PE over Economy for example).

Then of course there is price. For $1 extra I'd usually go for PE, but if $500 more, 1-way, I'd be sticking in economy every time. Somewhere in between is going to hold a fair number.