It wouldn't be a bad idea to use both. It could be nice to study a monolithic kernel-based OS (xv6) and a microkernel-based OS (MINIX). In 6.828, we read xv6, and we implemented parts of JOS, which is exokernel-based. Since exokernels are kind of like microkernels on steroids, I suppose it makes sense that we didn't focus on normal microkernels too much.