Airport prices are higher due to taxes, primarily (but not always) so for a 1-day rental from Portland to Reno the difference in taxes is probably not enough to worry about. The airport Hertz location has a MUCH BIGGER selection than any of the Hertz locations in town -- but, the standard advice holds -- reserve what you need. A minivan or a SUV, if reserved downtown, is what you'll get. At the airport if you reserve a car and hope for a SUV, you may or. may not be successful -- so if it were me, I'd shop around but likely just book the vehicle I need at the downtown location and see the time of going back out to PDX.