If it helps, I'll be staying at The Memphis Marriott Downtown and arriving midday. Is there public transportation or should I just cab it?
I suggest you take a cab. Public transportation isn't for the faint of heart around Memphis, especially if you've never been before. It's should be about $30 from the airport to downtown. If you plan on going anywhere else outside of downtown, I'd even suggest renting a car at the airport and just driving places from there.
Does your hotel have a shuttle service? Look into that. If not, then definitely take a cab. You're not going to find much public transportation 'round those parts!
Take a cab to and from the airport