Sioux Falls Southside Slamwich Showdown, What are the dates for the Sioux Falls Southside Slamwich Showdown? Getty Images/Dakota News Now (with permission) For the past month, there’s been a fierce battle going on to determine the best-tasting burger in downtown Sioux Falls, as that competition begins to wind down, another friendly little culinary competition is getting ready to begin from a group of restaurants in Southern Sioux Falls.