3
They are two different branches of the XOOPS CMS, but its highly highly recommended you use the 2.0.x branch for the itme being.
The 2.2.4 branch made some "drastic" changes to the XOOPS Core and although it was a good idea, it created some problems/bugs.
Bottom line, stick with 2.0.x. (I believe 2.2.x is actually newer, especially with the 2.2.5-beta in SVN)