Kindle (Wi-Fi) is a great choice if you already have a high-speed internet connection and wireless router set up in your home and do not need the added convenience of a 3G connection. 3G enables you to download books anytime, anywhere, while on the go. If you do not have Wi-Fi set up in your home, Kindle 3G may be a better option. 

Kindle Wireless Reading Device, Wi-Fi, Graphite, 6" Display with New E Ink Pearl Technology

Kindle 3G (Free 3G + Wi-Fi) is the easiest option because there is no wireless setup—you are ready to shop, purchase, and read right out of the box. Built-in Free 3G connectivity uses the same wireless signals that cell phones use, but there are no monthly fees or commitments—Amazon pays for Kindle’s 3G wireless connectivity. The added convenience of 3G enables you to download books anytime, anywhere, while on the go—without having to find a Wi-Fi hotspot connection. With wireless coverage in over 100 countries and territories, Kindle 3G is a great option for travelers.