In most cases, the upgrades are easier to snag on weekends at both downtown and airport locations. Anytime I do any kind of long weekend roadtrip, I rent from my local Hertz. We love Camrys for a long roadie, and have turned down several SUV or Merc/Linc offerings to get a Camry.

The only caveat is that a localized spike in demand (e.g., a large event at a hotel very near the Hertz location) could drain the location of cars quickly, even though the airport and the rest of the city has availability. That's pretty rare IME though...