What would break faster.throwing a pebble against another pebble hard or launch the pebble 50ft in the air?

Not a discernible figure. Your experiment does not take into account the surface, if any, the pebbles rest against, the inclination of said surface, the vector or force with which the tossed in the air pebble is thrown, or the height or the weights of the pebbles in question and thus the gravitation exerted against the pebble tossed or bashed.