[SalesForce] Use Account Fields in Case Standard Controller Visualforce Page

You can go with the Upsert Option. Upsert can be used in places where you are not sure if you'll have the Id of the Object and you want to insert/update the record.

If the object doesn't have an Id, it performs an insert and if it has, it does an update call.

   public pageReference SaveandClose()
    {
        try{
            upsert this.Test;
        }        
        catch(Exception e){
            ApexPages.addmessage(new ApexPages.message(ApexPages.Severity.ERROR,'Error creating/updating record'));
        }
        PageReference pageRef = new PageReference('/t');
        pageRef.setRedirect(true);

        return pageRef;   
    }

Also, as a side note, always have DML statements in a try-catch block since you might not know when you would run into DML errors from validation rules, triggers, etc.,

Rather than paraphrasing the difference, here is a great explanation, and I have have also added my own view.

A standard controller consists of the same functionality and logic that is used for a standard Salesforce page. For example, if you use the standard Accounts controller, clicking a Save button in a Visualforce page results in the same behavior as clicking Save on a standard Account edit page. If you use a standard controller on a page and the user doesn’t have access to the object, the page will display an insufficient privileges error message. You can avoid this by checking the user’s accessibility for an object and displaying components appropriately.

A custom controller is a class written in Apex that implements all of a page’s logic, without leveraging a standard controller. If you use a custom controller, you can define new navigation elements or behaviors, but you must also reimplement any functionality that was already provided in a standard controller. Like other Apex classes, custom controllers execute entirely in system mode, in which the object and field-level permissions of the current user are ignored. You can specify whether a user can execute methods in a custom controller based on the user’s profile.

See VisualForce in Practice for a complete guide.

Using a standard controller means that you will not have to write any tests, or maintain any code - that is the main benefit, but you are restricted to more basic functionality and logic, but this is fine if you just want a custom visualforce page for Create, Update and Delete. However, as mention in VisualForce in Practice you could use a standard controller for all of these features, then extend it with a custom controller if you wanted to do something more fancy - like a complicated calculation or have two or more objects within the same visualforce page.