For bus, Red Coach offers picks up on the MIA departures level and drops off in Orlando (downtown or near the airport). Their website shows upscale looking coaches with free wi-fi and movies:

http://www.redcoachusa.com

One other consideration is to consider your plans once you reach Orlando - if you drive up in a rental car, you will have use of that car to get around the greater Orlando area. Without a car in the spread-out sprawl of Orlando, getting around will be very expensive (taxis) orvery inconvenient and time-consuming (transit).