Car and Driver's plus/minus summaries were pretty good:
Prius Prime:
+ Lulls you into the slow lane
+ Soft Ride
+ Lots of Back Seat Space
- Oddball Design
- Lacks' the Volt's Range
- A tortoise on the road
- Minivan handling
= Sometimes the Hare wins
Volt:
+ A better EV
+ A better hybrid
+ Better handling
+ Better looks
- Small rear seat
- Costs more
= The best plug-in hybrid yet
Prius Prime:
+ Lulls you into the slow lane
+ Soft Ride
+ Lots of Back Seat Space
- Oddball Design
- Lacks' the Volt's Range
- A tortoise on the road
- Minivan handling
= Sometimes the Hare wins
Volt:
+ A better EV
+ A better hybrid
+ Better handling
+ Better looks
- Small rear seat
- Costs more
= The best plug-in hybrid yet