You are mixing things up a little bit. Darwin (the underlying kernel layer of MacOS X and the rest) is actually a hybrid between a microkernel and a regular kernel. There is a microkernel there, but much of the services layered on top of it are done as a single kernel. All of that operating within one memory space. So some of the benifits from a pure microkenel are lost, but a whole lot of speed is gained.

So from a security standpoint MacOS X is mostly in the kenel camp, not the microkernel one.