This study found that incline bench produced much greater activation in the upper chest compared to the lower chest. Muscle activation in both portions was similar in flat bench, however. Lower chest activation was also higher when using a wide grip, so make sure to use a variety of bench angles and grip widths for optimal chest development.
Source: Barnett, C., Kippers, V., & Turner, P. (1995). Effects of variations of the bench press exercise on the EMG activity of five shoulder muscles. The Journal of Strength & Conditioning Research, 9(4), 222-227.