I usually just add users to groups, because it seems simpler:
The default sharepoint groups, including permissions, are already defined for you.
You can add both fba users and active directory users to SharePoint groups.
There are some advantages to using roles though:
Roles can be added to sharepoint groups, while you cannot add a sharepoint group to another sharepoint group.
If you use your membership database in other applications, the roles travel with it. SharePoint groups are stuck in SharePoint.