Does anyone have any thoughts or guiding philosophies on when to extend a controller and when to create a controller extension?

One example that I'm working on right now:

• I have a customer portal for which I am creating a Visualforce tab to enable the customer to update his/her own contact record.
• I have created a custom controller called CustomerPortalUserCtrl that uses SOQL to grab AccountId and ContactId for the current user.

My question:  For the Visualforce tab to edit the contact record, should I extend CustomerPortalUserCtrl or write an extension for it?

public class CustomerPortalUserContactCtrl
        extends CustomerPortalUserCtrl {
    public Contact contact { get; set; }
    
    private Boolean editing;
    public Boolean getIsEditing() {
        return editing;
    }
    
    public CustomerPortalUserContactCtrl() {
        super();
        
        List<Contact> contacts =
                [SELECT Id, FirstName, LastName,
                        HomePhone,
                        OtherPhone
                 FROM Contact WHERE Id = :user.ContactId];
        
        if (contacts.size() > 0) {
            contact = contacts.get(0);
        }
    }   // public CustomerPortalUserContactCtrl()
}

public class CustomerPortalUserContactExt {
    public Contact contact { get; set; }
    
    private Boolean editing;
    public Boolean getIsEditing() {
        return editing;
    }
    
    public CustomerPortalUserContactExt(CustomerPortalUserCtrl ctrl) {
        List<Contact> contacts =
                [SELECT Id, FirstName, LastName,
                        HomePhone,
                        OtherPhone
                 FROM Contact WHERE Id = :ctrl.user.Id];
        
        if (contacts.size() > 0) {
            contact = contacts.get(0);
        }
    }   // public CustomerPortalUserContactExt(CustomerPortalUserCtrl)
}

One of my thoughts is this:  Extend the controller if I need to access the controller members in my new page; Create a controller extension if the only reason I need the controller is to initialize some internal variables for the new page.

Note:  My goals are complicated by the fact that we deal with Person Accounts, so using an apex:detail component would not work.  An apex:detail component with subject="{!$User.ContactId}" would render an Account detail page, which is not editable.