I am wandering what is the most efficient the getCmp or the up/down method?
The added value of the up/down is that it avoid the use of id/itemid.
The added value of getCmp is that you can evoke at any place any component.

Let says that I have something like:
Window (id/itemid = 'mywin')
----Button 'Save'
----Button 'Close'
--Form (id/itemid= 'myform')
----Textfield 'Register number' (id/itemid = 'tfreg')

The handler (onClick) on the save, obviously, will send the form data:
1) Ext.getCmp('myform').getForm().submit(...)
2) button.up('window').down('form').getForm().submit(...)

The handler (onClick) on the close, will close the window:
1) Ext.getCmp('mywin').close();
2) button.up('window').close();

In extension, I can do check on the textfield on the save like:
1) if (Ext.getCmp('tfreg').getValue !== '***') {...}
2) button.up('window').down('#tfreg').getValue !== '***') {...}