Hi Sebastian, that's a very good question.
I guess, it depends if you already own some Windows/Android devices then the choice is easier.
In any case,
Version - we always recommend the newest versions of CMS and clients.
Differences - Have a look at 'Feature comparison' section on this page http://xibo.org.uk/about/
One more thing, Windows client is distributed under open source AGPLv3 licence (ie it's free), Android client comes with built-in 14 days trial after which you need to purchase Xibo for Android licence for it (15GBP).
Android - Our recommended list https://community.xibo.org.uk/t/recommended-android-hardware/4340
Windows - there is some feedback, here https://community.xibo.org.uk/t/what-is-currently-the-best-player-hardware/678 and here https://community.xibo.org.uk/t/intel-stick-computer/3124