VS Au will get you Delta Economy Plus seating within the US, plus the other priority benefits as noted above. Ensure your Au number is in your booking and when you access your DL reservation online you'll be able to select economy plus seating. It's a benefit that is worth it. FC Au does also put you on the list for upgrades, but since ATL is Delta's prime hub it may be popular with DL's top tier fliers who come out ahead of you in priority, so don't expect it to happen.

Note that only internal flights that are connections to or from an international sector get lounge access.