All other things being equal, 7 plies tend to be stiffer and are designed to be so. Stiffer often means faster (quicker rebound, less dwell or vibration on contact). Plastic balls are slower so you may want a faster blade, depending on how you play, but there are fast 5 plies as well as fast 7 plies. Even fast 1 plies - thickness and composition usually play a bigger role than number of plies.