There is very much a trade off between 18's and 19's. 19's look better, come with stickier rubber (less range) but also have a firmer ride. Some people might prefer the 'sportier' feel, the car is pretty quick even in base trim.
The 18's are a good compromise between range and comfort for normal everyday use IMO, I'd probably even leave the covers on.
That's a good graphic, I hadn't realised how many Volt's and Prius PHV/Primes they had sold. PHEV's and hybrids in general seem like a missed opportunity now, their time has pretty much passed already.