Ok, since you asked. Here is a few of the Prius owners comments. These are not my opinions, just a copy and paste of their comments. Actually, several of them defended the Volt.
Here goes:
I considered a Volt, but chose the Prius Eco instead. Why? My commute is 95% highway, 70+ mph, 225-230 miles round trip daily, with no place to plug in at work. So I'd make it less than 1/4 of my daily drive on electricity, then the gas engine would kick in. The Volt gets in the low 40s on the highway, and I'm averaging about 51 mpg.
Add in the purchase price difference, and over time, the Chevy would cost me more. And that doesn't even include the cost of electricity.
I didn't consider the Volt for very long. I didn't even test drive one. When I was shopping, the Prius was far and away my first choice. My number one consideration was gas mileage. My second consideration was reliability
first of all, these are two completely different cars, and the choice should be easy.
but there are three important things imo:
1) interior room, front, rear and hatch.
2) typical driving patterns, are 50 miles of ev something you want and can well take advantage of, or would 54 mpg be better for you.
3) after a good long test drive of each, which did you like better?
Well the Gen4 is closer to 20-ft3 more space than Volt, with much more space in the rear seats and cargo.
In that case, consider the Prime plug-in as a better compromise between a Gen4 and a Volt.
I think the big problem with the Volt is the quality of the build. I remember watching a few reviews and even after a year, screws and other things were beginning to rust. I have had an issue with my previous Prius "C" but it was covered under warranty and there was no hassle.
I think it is a no brainer if you ask me. However it does depend on how you drive, where you drive but I'd probably still buy a Prius Prime if I were choosing between that and a Volt...
The main reason I went with a Prius was Toyota reliability over GM. My personal experiences. I will sacrifice bells and whistles over a "gee, we're not sure what is wrong with your car, but we will at least get it running" or "yeah I know your car is only 2 years old, but that part isn't covered under warranty and is a special order" type stuff I have been through it and I hated it. No issues like this with Toyota.