I don't know where you live, it may be that you're someplace where both SS and Herndon are a hike, but I wouldn't put my child through a couple of hours in the car each day in order to have access to a gym. If I thought my child did better with regular exercise, I'd take that time and money spent on gas, and find something to do afterschool in the winter that fufilled that need. If you're in Silver Spring (or even just picking up in SS after school) you could look at the special needs gymnastics classes at Silver Stars.
To be clear, I'd be willing to drive to Herndon if it was the only school that I felt suited my child, but if my child was offered a space or seemed a match for both SS and Herndon, I would choose based on commute and not the physical plant.