I think this part will blow your mind, they are going to let you Pre-Order the iPhone 5C. Preorders for the 5C can be placed with Walmart starting Friday 9/13 and must be picked up and paid for by September 25. Orders will be filled on a first come, first served basis. The iPhone 5S is not available for preordering, so you will have to go to Walmart on September 20 get the discount on that phone. I don not know if you will be able to pre-order online or if it has to be in the store.
Here is a comparison of the features for the New iPhones: