sp_WhoIsActive vs sp_BlitzFirst @ExpertMode = 1
They're just different scripts. 

Adam's sp_WhoIsActive is powerful, free, and copyrighted. I've used it for at least a decade, and I love it, and I still continue to use it personally. However, it's not open source, which means you can't contribute code to it, and you're not allowed to distribute it to others.

We needed something we could redistribute to our clients for commercial purposes, and let them distribute it to THEIR clients (because some of our clients are software vendors and service providers). That's why we wrote sp_BlitzWho and licensed it with the MIT License, enabling us (and others) to do just about anything with it.

They're both great tools. I still teach people how to use sp_WhoIsActive because most folks don't need the ability to redistribute code or make improvements to it.

About why I still use sp_WhoIsActive - when I troubleshoot blocking chains, I prefer sp_WhoIsActive because of its get_locks parameter that shows locking in objects. We don't need the ability to edit that code or redistribute it, so we haven't bothered building that logic into sp_BlitzWho.