The words are both used which I've said before. Rock in greek has multiple words to represent differing kinds just as in Chaldee and Aramaic. Chaldee has more than aramaic; but, then Chaldee is the root from whence comes Aramaic. Lithos is pebble sized, petros is larger still loose rock, And Petra is the general term for masses of rock - large masses of rock, thus it's primary definition as cliff, mountain, bedrock, etc. These are states of being just as expressed in the language. One word for loving the pet, one word for loving the wife. One word for little rocks, (lithos), One for bigger rocks and boulders, petros, and one for Massive rocks like cliffs, sepelchres, mountains and bedrock. States of being small, medium, large. This is the way they were defined and this is the way they were used by the Apostles. I'll have to wait till I get back home to post the textbook definitions.
I might also note, that petros is a specific term denoting size of rock. Rock is an english generic that can be used to describe any type of stone from bedrock to tiny gravel. It lacks specificity where the greek words do not. This is why Greek is such a good language to work with. One can't hide as easily behind generalities in hopes of obscurity. One is forced to alter the language or lie about it to gain any pretense. Here we find others trying to obfuscate using the generality and vagueness of another language because the Greek won't allow it.
John 1:42: "And Jesus looking upon him, said: Thou art Simon the son of Jona. Thou shalt be called Cephas, which is interpreted Peter."
That Greek word "interpreted" = "ermeneuetai" and it just as easily could be rendered "to translate." Petros is a translation of the name that Christ gave to Peter: "Kephas".