Usually, AD group memberships is controlled without the need for confirmation from a DBA - for a very good reason, since the SBA should not be concerned about a member being added/dropped from an AD group.
But that's exactly the reason why a the sysadmin priv should not be granted based on a AD group: the DBA team won't notice any change (at least not with additional effort and mostly as a reactive task, not a proactive).
The argument, that a specific user account would add additional effort in case of leaving the company is rather weak: if the account is disabled at the AD level, how would a user be able to connect to SQL Server (except using a local Windows account that should have been detected before...).
I, personally, advocate for user based sysadmin privs instead of group based. Just because of the harm that can be done.