I also called USCIS, the woman said GC is the most important.

Because I kept asking about the on-line status "approval notice sent". She finally said the welcome notice and the GC would be all for an approved case. It sounded that she was also not very clear of the documents that should be sent and received.

BTW, in the automatic message before talking, only welcome notice and green card were mentioned, I never heard approval notice in there.