There are a number of national rental car companies. Usually the best strategy is to shop around. Rates may differ between the companies, different offices of the same company, even between different agents of the same office (it happened to me). Although the airport may be the closest location, there is an additional tax and some other charges if you rent from the airport. (Thus it is usually more expensive)

If you are under 25, most of the companies have surcharges (some are unreasonable, e.g. $80/day). The only company that does not apply a surcharge, given you're over 21, is Enterprise. However, they have mileage limits. In short, shop around and if you're under 25, make sure what the surcharge is.