For IDPA, your ammunition must:
1. Function the gun and
2. Make the 125,000 power floor for SSP.
MOST people find that a heavy bullet at low velocity has less FELT recoil than a light fast bullet in an uncompensated gun.
But I load a 135 gr 9mm for my Plastic M&P and find it comfortable to shoot, so if you want to save money with lighter bullets, you will probably be ok.