Voltage regulator device selection
The primary advantage of an NMOS transistor is that the higher mobility of electrons means that you can use a physically smaller transistor to pass a given amount of current. Of course, the disadvantage is that if you use an NMOS pass transistor then the maximum \$V_{GS}\$ you can provide is 1.3V, but if you use a PMOS transistor the maximum \$V_{GS}\$ is 2.5V. Since you want to sink as well as source current the typical way of doing this is to add a fully complementary buffer (i.e. a big inverter) at the output of the op amp, with a PMOS to 2.5V and an NMOS to ground.