The convenience of the M4 or M4S is with the use of a AA battery as opposed to the 1/3N lithium battery used in the PRO. Otherwise, the M4/S has 80k hours of battery life while the PRO has 50k hrs.
I use a PRO on my duty rifle, and I have the M4S on my primary SBR at home. The QRP2 mount that comes on the PRO is sufficient and good quality, and given the fact that it's included with the optic, it's a great value. The M4 is $300 more for the same mount. I don't exactly think it's worth the extra cost. Both are die-hard reliable.
I personally like my M4S better than the PRO, but it's because of the LaRue mount I have on it (I got my M4S without a mount), and because of the AA battery. I'm not the biggest fan of the factory QRP2 mount because it has a tendency to loosen up under vibration. The M4S is a frickin' tank, and it has the weight to prove it. It's NOT a light optic. The PRO is much lighter.
Have you ever considered looking at a T1?