That depends on what "side" of GATT link are you looking for;) BLE is asymmetric on pretty much every layer and NUS need GATT Client and GATT Server. ble_nus is GAP Peripheral + GATT Server (usual "embedded" option, that's why it is kind of "default"), ble_nus_c is GAP Central + GATT Client option (usually handled by device with higher power budget = bigger battery).
