Orlando Sanford International Airport (SFB) is a secondary airport in a town north of Orlando. SFB primarily serves international charter flights and domestic discounter Allegiant Air. It's about 48 miles from SFB to Epcot (to pick a specific location within WDW).
Other domestic airlines, including the "cheaper" ones, fly into Orlando International Airport (MCO). It's about 23 miles from MCO to Epcot.
(Actual distance depends on the route you choose.)
If you're renting a car, the major difference is that you'll spend an extra 25 minutes each way if you use SFB. You'll gain back some of that time because SFB is a smaller airport, so you should be able to get in and out more quickly. However, if you plan to rent a car, you'll have fewer choices. For example, Simply Wheelz by Hertz is only at MCO.
As others have noted, DME is not an option at Sanford, and limo/towncar/van/SUV services are quite expensive because of the distance.
In summary, if you plan to rent a car anyway, SFB is a good option if the flights are cheaper. But if you're not planning to rent a car, you have to factor in the greater ground transportation expense -- and flying into SFB may no longer be a bargain.