The earlier statements with regards to the lack of the third row was that it required the Prius v to have a higher overall cost and also require having the Lithium Ion battery pack, (Smaller pack, but same amount of battery power) as well as bulking up the car a little more. It was believed that the overall cost would not make it a very sellable car in the US, especially with our current exchange rate.
As you pointed out yourself, it really comes down to what you and your family want and what is the highest priority. Economy, space, purchase price, etc. For us, the additional space was handy and the fuel penalty was almost negligible. I know some who really complained about the horrible mileage on the v, but it amounts to a couple of cups of coffee a month.
OP: you are a 2 car family, so it seems you have the option to tailor each car for different tasks. Rental is for the rare days your two cars do not manage.
