*Both.
Brother, even most protetstants are now admitting Peter/Cephas/Kepha was the rock.
When Jesus first spoke to him - Gospel of John - He tld Simon Barjonas he would change his name to Peter.
"Peter" as a name had never before existed.
When God changes your name, it signifes something rather important,no?
As a word, Petra verses Petras did exist. The word Jesus called Peter means "little stones, or pebbles". Upon the "Big ROCK, Boulder" of the truth that Peter spoke concerning Christ, Jesus would build His church.
Now, as to what "most protestants" do or do not believe 1) What statistics prove this? and 2) What does it have to do with the price of tea in China? Most Protestants are like most Catholics these days and do NOT know Scripture. So, who cares what an uniformed decision says?