In fact, in 1 century only "petras" was used. "Lithos" (or is it "lythos") belongs to classical Greek.
More importantly, if Christ wanted to contrast the smallness of Peter with the solidity of some other rock nearby, he would not follow up by giving that "pebble" the keys to the Kingdom. Nor would Christ respond to a profession of faith with an insult. Nor would He repeat the commission following His resurrection in the "feed my sheep" verse.
And most importantly, Christ, who could foresee the future, would not have made such a critical error that could possibly be misinterpreted. If we are to believe that Christ's message was clear and undeniable, then the proclamations he made to Peter MUST mean exactly what they say.