If you are planning on purchasing extra items such as a damage waiver or insurance, you should also comparison shop those rates.
Make sure their mileage policy is favorable to you so you can avoid extra charges.
Airport fees can be quite large so you might see if it is worth it to use an off-airport facility. However, if the off-airport company picks you up at the airport, you'll probably still have to pay some of the fee.
You can check their national reservation centers and their local offices; one may be offering a better deal than the other. ASK, if this is the best rate they have available.
Make your reservation as soon as you have decided on plans. Most Rental Car Companies increase their rates as reservations come in and their fleets become booked. Also, certain classes of cars will sell out and you may have to end up reserving and paying for a larger vehicle than you want.