"Office365 Outlook SEnd Approval email" was the original Approval method.
This year Modern Approvals were released with more advanced features (the possibility to send a comment is one of them).
Microsoft Flow Team is pushing hard to enhance Modern Approvals; so I guess "Office365 Outlook Send Approval email" will finally be deprecated (please note I am not talking on behalf of Microsoft Flow Team, it is just a personal opinion).
In the meanwhile, the old Approval method has some features that are not yet available at "modern approvals" (or at least I haven't found them), like the option to modify the button labels, have more than 2 labels... there is also a workaround to include HTML body inside the email approval (the workaround works in the old Approval method but not yet on the Modern one, I guess).
Both Approval Methods (Modern and Old) are based in https://docs.microsoft.com/en-us/outlook/actionable-messages/
However, I do not think you can build actionable messages via Microsoft Flow in a customized way.
Hope this helps