Although many of us here agree Volt is better, I have a sneaky suspicion that the Prime may upend the Volt on sales this year. Simple brand recognition--with an improved (if deficient) plug-in offering.
One of my workmates bought a Prime. He moved "up" from a regular older Prius. We plug-in owners (there are four of us), shook our collective heads. We have 2 Gen 2 owners, 1 Gen 1 (me), a Tesla, and now a Prime. We all drive a LOT for work, but the Volts still smoke the Prime on lifetime MPG (and of course the Tesla smokes us all--but sometimes he breaks out his VW due to range anxiety, so could be more of a wash). After looking at the Prime's interior, I was shocked at how "Gen 1" it is. White ceramic accents, 4 seats, channeling the "gee wiz" tech factor (with form sacrificed).
In the end, the more with plugs the better. I don't mind a little healthy competition on "who gets the best" MPG. Maybe someday Prime owners will step UP to a *Gen 1* Volt after getting annoyed at such little EV range.