We have not performed any true comparisons between the two methods, but as memory consumption goes I would expect the DB method to require less memory, since some of the UI components are not created and/or initialized for non-UI documents. However, I would also expect the difference be ignorable (percentage-wise) on anything but small documents. From that point of view, I would not make it the decision driving reason. Whether to use the DB method or the UI method (OpenAndActivate) should be driven by purpose of the add-in. If there is no other UI involved, I would definitely prefer to use the DB version (OpenDocumentFIle). Not only it is going to be slightly faster, but it is guaranteed to work regardless of whether or not Revit operates with its UI present or without. That is naturally only true if the addin is implemented as a ExternalDBApplication (i.e. not regular ExternalApplicatio which always requires UI).
Cheers
Arno?t
Arnošt Löbel