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.

Quoting from the Xv6 web page, "Adding multiprocessor support requires handling concurrency head on with locks and threads (instead of using special-case solutions for uniprocessors such as enabling/disabling interrupts) and helps relevance."

MINIX didn't have multiprocessor support until MINIX 3. Unlike MINIX 2, MINIX 3 is not designed as a teaching OS.