They are very different animals, though some of the things they do overlap. Houdini is vastly more powerful in more areas, being as it is, an entire DCC application rather than XP, which is, at its core, a particle plugin for Cinema. Both have their place, XP is amazing, but Houdini is so extremely reasonably priced, sale or not, I'd say get both ! Houdini is a total powerhouse, but you can't beat the convenience of having some of that sort of power direct in Cinema, which is what you kinda get with XP.
CBR