Circle wards can hold for ever without vis investment especial if the circle it self is build instead of just drawn while Aegis need more time, need to be redone each year and need vis investment.
Also I find it a bit strange it a creature of might can enter the Aegis but many if not all of its powers would fail at the very moment it enter the Aegis.
Let say a Might 25 creature fly into a lvl 25 Aegis that only have a penetration of 20, would't the flight power without penetration end in the second the beast enter the aegis?
About Ghost and Animal there I missremembered as the warding circle spells specifical say it only affect creatures of magic might. But there for sure also Ghost with Divine Might, one type is mentioned in RoP:D 133: