I was looking seriously at the M&P for IDPA and I called and asked S&W the same question. The Pro has a trigger job and the 9L doesn't. The Pro has the fiber optic front sight, and one other things...can't remember. You can call S&W and ask.
If I can find a M&P 9mm Pro at a good price, I may pick one up and see how it does in IDPA. Right now, though the Cz75 Shadow is a sweet trigger and I will use it until I get better at IDPA meets.
Right now the M&P Pro 9mm is hard to find, but that is the better gun.