Welcome to MobileRead, Jose.
The kindles whispernet is "built in". No subscriptions.
As for which is better, ..........I love my Kindle, and Sony people love theirs. I think they are both great devices.
If you are in the US,..........Amazon has a great selection of books. Also, you may want to make sure using their map that you get whispernet where you are. Here is their page showing maps, etc.
http://www.amazon.com/Wireless-Readi...sa_menu_kdp2i3
The Kindle also comes with a 30 day unconditional guarantee, so if you buy it and don't like it, you can return it paying return shipping charges only.
Whichever one you get her, she'll be thrilled.