I've had better luck with Hertz than National. It really depends on the location and who is assisting you. Most of my rentals are in Northern California and most of the Hertz locations know my rental patterns which most likely contribute to that.

With that said, National tends to have lower mileage vehicles and more Premium and up vehicles to choose from. They have fewer luxury and up models and specialty cars than Hertz (i.e. prestige, adren., and dream cars) however you'd probably have a better chance getting a higher luxury car for no up charge by asking at National. Fewer to pick from but potentially easier to obtain if that makes any sense?