There are two ways in which you can get XBMC installed on the Pi: OpenELEC and RaspBMC.
Here is a head-to-head comparison between the two.
Installation
OpenELEC only has a limited way to install. You need to download the installer, unzip it, and run the command to copy the files onto an SD card.
RaspBMC has more ways: Download and burn the installer on SD Card, Download the image file and burn it, or copy the installer files into SD Card and dowload during first boot.
OpenELEC: 6
RaspBMC: 8
Usability
The screen performance are a lot better on OpenELEC. It boots faster and it loads program quicker than RaspBMC.
OpenELEC: 8
RaspBMC: 7
Features
RaspBMC has additional features such as using a remote TV to controll it. It also have automatic updates running in the background
OpenELEC: 6
RaspBMC: 8
The Winner: RaspBMC
If you want features over performance (speed) then you should go with RaspBCM, but if you want a simpler setup for your media server with quicker performance than OpenELEC is for you.
Other, more elaborated argument can be found here: ttp://www.linuxcircle.com/?p=488