You will get a Prius if you reserve one. Hertz will take it out of their inventory if you reserve specific models (including other ones). The Hertz Priuses are not that nice trim wise, but I rent them for short rentals if I expect to drive a lot since the gas savings will often cover a large part of the rental cost.

This is especially true for weekend rentals at airports like IAD where the total cost all-in with a good promo code can be as low as $17/day. After factoring in the 500 Marriott points earned and the gas saved for driving 350 miles, my one day rental had a marginal cost of about $3.