I´m not exactly sure what kind of impact these two things have on kernel design. Are servers more flexible because of running in user space, for instance, and does adding a module require a new build of a kernel?
module runs at the same memory space than the kernel and a
server runs in a different one.
module can surely be added/removed on-the-fly (linux kernel does that), any misbehavior caused by it may affect the entire system, whether in the micro-kernel architecture, only the
server gets compromised.