The most important difference for you is that you don't want to check out the entire repository anymore: unlike CVS, Subversion would leave you with all tags and branches as separate files on your hard drive - which totals in about 1.4 GB of files at the moment. Instead, you will specifically ask for the "trunk" only - that's what you already know as the CVS root now.
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To get a copy of the current Haiku tree (and Haiku only), you would write something like this: