Also, IMHO, both games are a lot of fun -- albeit with a large and steep learning curve, and if you like one, you'll probably like the other.

They are also slowly becoming more similar in some ways. Originally Dwarf Fortress had one managing a band of dwarves on a fixed map, but then added a single-character adventurer mode where one could roam around the world. Originally Cataclysm was a single-character game where one roamed around the world, but added the creation of faction camps recently, where one can manage a band of people at a fixed location.