Generally (but not always) prices will be cheaper if you rent from the airport, simply because there is a lot more competition at the airport.
At the airport there are many companies with large fleets of vehicles to try to cater to the millions of visitors arriving each year for their Disney/Florida vacations. There are a few onsite companies with much smaller fleets to handle the much smaller group of people who want to rent for a few days so they can go offsite and visit other Orlando attractions , the beaches, etc..
The goal of any rental company is to get as close to sold out as possible everyday (a car not rented out isn't bringing in revenue, better to rent it for less money than not at all). If cars aren't moving as predicted, prices go down and conversely if vehicles are selling out (or trending that way) prices will rise. This can happen at all locations (local and airport) but due to the volume of rentals at the airport, prices can change daily and sometimes several times per day as the pricing software tries to adjust. A smaller onsite location generally doesn't see these price fluctuations as much since it has a much smaller fleet and a much smaller renter base.