I want to install XBMC on my Raspberry Pi, but I do not know what OS to choose.
There are three:
What is the most efficient of the three? Which allows you to continue to use the Raspberry Pi as a classic Linux system?
For normal users I would say they are somehow similar, and the differences are not that huge. However, for advance users they are different. OpenELEC has a huge disadvantage for people who like to further customize their Pi since the system come as a read-only image, e.g. you cannot disable or enable services and you cannot even change the root password without having to do a lot of work on your computer to modify the image. However Xbian and RasbBMC uses a normal system partition that can be mounted as RW and changed from within the Pi. Both Xbian and RaspBMC are made for the Pi only while OpenELEC is not, so I find the attention of the developers are more targeted to add features and customization that fits the Pi the best. With Xbian and RaspBMC they made it easy for users to install other packages and applications (for torrent and other stuff) so you can do your downloads from the Pi itself without needing to use another PC. RaspBMC currently has only one developer as far as i know so don't expect much development, Xbian on the other hand has a few so you should see bug fixing and new features made much faster than RaspBMC.
I like that we have many options to select from, since that mean you will find what you want in one of them and also more features, enhancements and ideas that they can borrow from each other ;)
One last thing, you are not choosing between OSs, they are based on stripped down Linux that boots right into XBMC, however, they are more like distributions.
Note: I have used Xbian for sometime, currently using OpenELEC and planing to switch back to Xbian because of the many things that I cannot do or change easily with OenELEC.
You forgot GeexBox.
All of them are approximately the same, they use latest build of XBMC and the features are mostly similar. Raspbmc has a bit annoying tendency to update itself at the most unpredictable moments, others -- not so much. OpenELEC and XBian are (in my opinion) customized a bit heavier than others, that makes them a little snappier.
Regarding using as a classic Linux -- yes, there's a linux inside, but using
apt-get is discouraged (at least in RaspBMC) and may lead to a broken system. So as long as you don't install and upgrade packages -- you'll be fine.
Or you may try to install the plain old Raspbian and XBMC on the top of it, it might be a tad slower because of more running services, but you could watch movies and have a good linux experience at the same time.
I have had XBian running for a while now.
OpenELEC is built with the idea to fit multiple devices and therefore is not limited to the Raspberry Pi only. XBian and Raspbmc are only for the Raspberry Pi.
I also like the way XBian updates. Perhaps it is because I am Dutch and there are a lot of Dutch people using XBian and discussing it on a forum called tweakers.net.