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. These rules are meant to be the primary focus of a campaign with "adventurer level" action as a sideline.
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. The rules have been reprinted in Ultimate Campaign, but not in significantly more detail. They are suitable for settlement building as an adjunct to a normal campaign.