I want to create my custom toolbar at the top for explorer and inspector window. I want to make it ALWAYS visible. So which is the best approach, to use a custom task pane or a form region? Which best? What are the advantages and disavantages for each one?
The toolbar should be able to contain buttons, text fields, checkboxes, drop-down lists, etc.
There is no layout at the top for Outlook form regions.
Custom task panes is an Office-wide feature available in all Office applications (most of them to be precise). When you create a custom task pane for Outlook, the custom task pane is associated with a specific Explorer or Inspector window. Explorers are windows that display the contents of a folder, and Inspectors are windows that display an item such as an email message or a task.
Form regions are custom pieces of user interface that can be used to customize a standard form. They are not designed to customize the Explorer window, they are designed for item's forms. And created and designed only for Outlook.
You may also consider using Advanced Outlook view and form regions. They allows placing a form region at the top.