I would use the first. The latter was created during the early days of the EHR and I actually removed it this afternoon. The EHR module contains code for this project that is not part of the Labkey core. I'm trying to make sure there's minimal redundancy, but there still might be some.

Labkey overrides the default Ext combo w/ LABKEY.ext.ComboBox, so xtype: 'combo' should use their extension.