Going forward I prefer taking a Quad core over dual, it gives flexibility in the future and by no measure are Phenom II cores slow. For most gaming though the i3 2120 will be as fast maybe a few % faster.
If the computer is mainly for gaming I would go with Intel. Right now AMD really doesn't have much that can compete clock for clock with Intel's CPU's. Clock for clock Intel will be faster in most games. Depending on your budget I would go with an I3 or I5.