Behind the scene, update means delete the existing one then add a new one so add in theory should be better, especially when you are dealing with millions of documents.
Adding only leaves you with "duplicated documents" meaning there will be at least two or more documents in the index that have very similar contents (or minor differences) The definition about a dup is varied based on the data domain and business needs so don't take it personal when someone says "your definition of a dup is wrong"
For example, if a document is about a person and his/her address. Version 1 has one address, version 2 has a different address. Ask yourself, what does your business want to do with this? If it only prefers the most up to date address, then you need to do an update, not an add. If it wants to keep a history about one's addresses, then you need to do an add.