The only reason to get a wireless keyboard is if you don't want it tethered to your computer. Outside of that, they function exactly the same way... only one has batteries that can run down which need to be recharged and as others have mentioned, requires a certain level of Bluetooth in order to communicate.
In a nutshell, wireless isn't better and wired is always more reliable. Since keyboards tend to stay put, the fact that they are tethered isn't a big deal. Mice on the other hand can be obnoxious if wired depending on what you use your computer for.
If you have no compelling need for a wireless keyboard, stick with wired.