Make your downtown reservation - costs you nothing but establishes a promised price and puts you ahead of walk-ins.

I find it interesting that you pay extra to pick a car up at the airport. When I booked our SUV for our recent trip to the US I compared prices and found, using the Dutch sites at least, that there was no difference in price whether we picked it up at the airport or at an office in the city.
I just checked again and found that still to be the case.
We booked through Alamo, using their Gold plan, and booked an intermediate SUV - yes they tried to get us to take a free upgrade to a bigger SUV and no we stuck to what we'd ordered. Went down to collect the car and found at least 20 to choose from, so it wasn't that they didn't have any! Didn't find the car (a Nissan Rogue) to be that thirsty to be honest.

hetismij,

there are a big difference in tax rates and airport fees.

For example, in my local city the quoted base rate will be the same at most locations but at the airport location they hit the out of towners with a $5.00 fee/day to pay for the basketball stadium that was built 5 years ago.

hetismij - As eschule mentions, airport fees and taxes run the gamut and can more than double the price of the car rental.

I recently booked a three week rental from an off-airport location (Denver). The off airport location rate was HALF that of the airport rate - same class of car, same rental company.

I'll collect the car off airport (I'll take a shuttle which will run about $19) and return it to the airport upon departure. A tiny hassle, but a savings of about $700.