The Raspberry Pi is technically just a embedded linux platform, but it's low price and features make it perfect for a PVR backend. In addition to this, since a lot of people will hopefully be using R-Pi's as backends, it will make it easier to streamline setup guides and provide help due to most of the hardware being the same for everyone.
↑VU+ (and other Enigma2 set-top boxes) aren't actually software you install, but rather a PVR/DVR set-top box itself that can interface with XBMC. If you have a Dreambox, VU+, or other Eminga2 box, this is the option you will want to use.