The mechanical performance of a typical surface course mixture, modified with two different plastic wastes, both via the wet and dry processes, was evaluated in this study. Water sensitivity, rutting resistance, stiffness modulus, and fatigue cracking resistance tests were used. The results obtained indicated that the polymer-modified mixtures showed similar or improved performance when compared with that of a conventional control mixture produced with a harder virgin grade bitumen, not always available, or available at higher costs, in several countries. Thus, modifying asphalt mixtures with these plastic wastes can be an economical and ecological alternative for paving works. Moreover, the mixtures produced via the dry process showed increased water sensitivity and stiffness modulus properties. This holds out new possibilities for use of polymer-modified mixtures, especially in developing countries, because it widens the possibility of using locally available bitumen, of variable quality, for producing mixtures with higher performance. This can be achieved at real scale with no major extra costs because the dry process does not require modification of typical asphalt plants.