I'm going to Lisbon, Portugal in 2 weeks. Last night I looked for a car on Hertz and Avis, both with identical dates and times and offices just a few blocks away from each other. Hertz cheapest offer was $70 more than Avis and the GPS was not guaranteed! I'll try National now and compare. Interestingly, I did not see any difference in price making the reservation as a "guest" or using Amex Plat "special" discount codes.