To my way of thinking they are not even the same kind of thing. Pligg is a "digg" clone/workalike, while Joomla! is a general purpose CMS framework. I realize that Pligg calls itself a CMS, and it is, in a literal sense, but it is not what I generally picture when I think CMS.
I think one user's description of Pligg is pretty accurate:
To me, one's a fruit and the other is a vegetable.