I certainly don't know it all, that's why I'm a member here and many other forums and Facebook groups....but, the way I understand it is this....both SimonK and BLheli were developed for multirotors. SimonK is an older way to go for multirotors. I've used SimonK firmware in the past with great success. Then comes along BLheli. I've used the BLheli firmware with great success. BLheli is constantly evolving and updating, which is only making it better. I have 4 multirotors, ranging from 180mm up to a 770mm, and all 4 have BLheli. I love the simplicity of the Configurator.
My two cents.