you should avoid the classic portal if at all possible, i actually don't believe there is anything left that you can only do in the old portal. the new portal can display and deploy both ASM and ARM resources (Please avoid ASM resources, when possible).
The external user function you refer to is now "Guest User" you can add these accounts to your AAD and then grant them permissions. i would use the portal to assign permissions and remove the old classic permissions.
there is no comparison guide as they moving more and more to the new portal and Microsoft encourages not using the old portal if possible(Its still supported).
you also have aad.portal.azure.com if you only need to do AD tasks.
Anything you deploy via CLI and powershell will be shown in the new portal