The cost would be about the same, probably a little cheaper, except if you hit traffic, b/c the taxi has a meter, while the car service charges a flat fee.
There is a cabbie who occasionally posts here who says that any cabbie at MCO has the option to give you a flat fare if you ask for one, but IME I've never found a cabbie at ANY airport who would do that, except in cities where cab service is zoned.