For the V3, you must use the new app to set it up. After that, you can use either or both.
For the V2, you can use either or both.
They’ve already told us that the classic app will be going away eventually, but they haven’t said exactly when. For now they have suggested that those who already have a system set up with the classic app continue to use it.
If you have Samsung smart appliances or televisions, you will have to use the new app for those devices.
See the following thread (the topic title is a clickable link)