I've only read the rules for Kingmaker, but I've heard that Birthright is similar in many respects. How is the 2nd edition setting similar to the Pathfinder setting? I know they are both about kingdom building, but how does each setting approach the actual mechanics?
Birthright is a larger scale system designed for interaction between multiple mature countries. It's set in a particular setting and has magical "bloodlines" of the regents as a core conceit. Domain development is done in turns, and you can construct some general high level holdings, conduct trade and war, etc. A fully 3.5e'd version of Birthright has been created by the community at birthright.net.
Kingmaker is mostly a specific adventure path with some specific rules about carving a new civilization out of largely-unsettled areas and being sustainable. It has a more micro focus - exploration and then construction down to the building level ("I build an inn this turn!") Also keep in mind these are supplemental rules in an AP; there's 9 pages of kingdom building rules, then more later on mass combat and exploration, but the core is small.