One thing that I think is important to consider when deciding between SharePoint groups vs AD groups is whether or not groups of users need to be re-used across site collections.
A SharePoint group is bound to a single site collection, whereas an AD group can be used across many site collections.
Say for example you have 10 site collections, and five of those site collections will have the same set of users assigned permissions, while the remaining five will all have a unique set of users.
For the five that all require the same set of users, it would make sense to use AD groups to manage the users.
For the remaining five, you could either create a SharePoint group within the site collection, or create five distinct AD groups.
Generally I prefer not to have 1-1 mappings of AD groups to site collections, so for the last example there I would go for the SharePoint group option.