I wouldn´t recommend to buy a raspberry pi. Even less if you want to be prepared for future verions of XBMC and movie playback!
If you want to be save for the future, I would recommend to buy a NUC and possibly install OpenElec or XBMCbuntu (both are easy to install). A NUC is more expensive, much more expensive than the RPi, but it´s the more powerfull hardware and you can install every system you like...no need to take care of OpenElec, XBMCbuntu, Linux with and XBMC or Windows with XBMC.
For explanation..the differences between XBMCbuntu, Linux with XBMC and OpenElec are:
- XBMCbuntu: Ubuntu with preinstalled XBMC, as flexible as "Linux with XBMC", but XBMC is already installed
- Linux with XBMC: You can take any Linux you want (if you are able to...i would recommend Ubuntu) and install any XBMC you like (even nightlies). Given that you have Linux knowledge
- OpenElec: OpenElec is it´s own distro on which XBMC is also preinstalled, but not that flexible as the others.
So if you don´t want to do some exotic things, I would prefere OpenElec. If you want to be a bit more flexible, then I would recommend XBMCbuntu. If you have Linux knowledge...I suppose, you don´ t need to ask for the version
Windows can be taken, too...if you don´t want to fiddle around with Linux.
Greetings