[gregdunn]

Disclaimer: I work at AWS, primarily in the compute space, but I'm not speaking in an official capacity.

I would say they fulfill different purposes. The SSM agent has quite a bit of additional functionality, even within the Session Manager portion. It's more of your solution for online, general day to day access.

Serial console will let you fix issues when you have lost the ability to boot an instance, or network connectivity has failed. When SSH or Session Manager are available, I personally would opt to utilize them over the serial console. But if I have an instance that I can't reach via those, am unable to replace it for whatever reason, and need to bring it back online, serial console would be what I would reach for.