If they're at Petsmart, they're both. 99 percent of store bought "midas" or "devils" are a mix of both species. The only way to get a pure fish of either species at this point, is pretty much by buying from a very reputable breeder.
The differences are minimal. Personality and size-wise, they're basically the same. RDs tend to be more long and pointy looking, often with bigger lips. Midas are beefier, with stockier bodies and snouts. Personally I prefer Midas.