Hard to answer this precisely. The simple answer is that set necks (Epic) come standard with volutes, while thru-necks (Essence) do not.
It's not the overall strength of the neck itself, but rather that the thru-necks - which require much longer pieces of wood - actually use a scarf joint to attach the peghead to the neck, and then also have a bunch of laminates (top, bottom, and sides) that reinforce it.
There was a nice
discussion of this recently, with some photo references, and hopefully someone will come up with the link (oh, i guess i just did).
If you're willing to pay the price, you can get either a set or thru-neck with the opposite configuration. Personally, I also find the volute to be a bit distracting.
