If you want to make sure to get the iPhone 5c/5s as soon as it is available, then you will definitely want to preorder it. This way you can make sure you get it on the September 20th release date. While it will also show up in stores by that date, unless you are one of the first in line, you might not be able to get your hands on one as they are expected to sell out fast.
You can preorder your iPhone 5c/5s through the Apple Store, or other retail outlets such as Wal Mart or Best Buy. Wal Mart is the most cost effective as you can get a new iPhone 5c for $79 as long as you opt in to the two year contract. If you get your iPhone 5c through Best Buy it will cost around $99 which is the same price at the Apple Store. These lower prices are for the 16 GB version and go up from there for the 32 GB edition.