Sunday is absolutely a better day! Saturday is Mercy Heath Systems Co-Worker Appreciation day, which is a special event with ERT for those who work for the hospital. It's one of the largest employers in Springfield, so stay away Saturday (we'll be there on that day, though, as my wife works for them and we get to enjoy the ERT!). So go Sunday, absolutely.
Generally, lines are short for everything from here through the end of the operating year. Don't worry about it and enjoy your day! BTW, the water rides are open through October, but they open at 11:30. Lost River of the Ozarks, an Intamin rapids ride, is the best of the three. Also, take time for the cave tour; it's really a highlight of the park.
Good to know I should avoid going on Saturday. I went to the park two years ago but that was on a weekday in October. I usually avoid theme parks on weekends, hopefully I'll get to ride Outlaw Run ten plus times like two years ago.