Install both CMS's - play around with them and then when you come across your first problem/question/query ask in the respective forums.
Usefully for me I had an identical request for both joomla and XOOPS when I started - just a simple one about posting news articles.
Within the XOOPS forums I had four answers within three hours and ended with over twenty replies and helpful private messages.
With the Joomla forums I'm still waiting (this was three months ago).
It's the community that makes these open-source programs and it's the community that drives them forward.
For me I know that I can get answers and help for anything I want, with Joomla I know you'll get very little help without having to flash cash before anyone listens...