Both are full-featured free and open source web applications with which you can realize a very diverse variety of projects.
Both are written in PHP and use a database such as MySQL
Both can run on standard, inexpensive hosting.
Both have large & vibrant communities
Both use the Smarty template engine.
Different
Tiki is more centralized. Tiki is the all-in-one model while Xoops (like Joomla!) is the small-core-and-add-what-you-need model. Each model has its pros & cons.