Store Spray Tan

I always wondered what was the difference between store brought or salon spray tan. Is there a different ingredient that they use? I think a person would just have to buy every spray tan out there to see which one is the best. There  are even websites were you can order them online, but if you order them online you will not get to try them out first. In the stores I usually like to read them over to see what exactly I am getting. I like to compare the prices to make sure I am getting the best deal on my purchase.