I would highly recommend taking the train. Fares can sometimes be found from $12 or so (though $29 is typical); connections are available to/from the Miami airport (via Tri-Rail) and the Orlando Amtrak station (bus, taxi, etc.). Actually, if you are WDW bound, you can just get off at Kissimmee instead of Orlando.
Miami departures come at 7 and 10:35 a.m. daily.