I don't see where anyone broke it down, so I will. before I start I will say both are good optics, but the comp m4 is hands down better in most regards (it should be considering the price)
aimpoint pro- entry level optic for professionals, lower price point, a lot of value for the dollar.
weight 11.8 ounces
battery life 30,000 hours
waterproof to 45 meters (150 ft)
battery type 2L76 or DL1
6 daytime brightness settings and 4 nvd settings
of the two the pro is much cheaper
Aimpoint Pro Optic Sight Aimpoint Pro Optic Sale 12841
the aimpoint comp m4/m4s the best of the aimpoint comp line, boasting a very advanced electrical system and being probably the most robust sight you can get.
weight 11.8 ounces
battery life is 80,000 hours
waterproof to 45 meters (150ft)
battery type - AA
9 daytime brightness settings and 7 nvd
Aimpoint CompM4s Sight Aimpoint CompM4s Sale
they are the same size and weight, but I will say, holding the aimpoint compm4 sights and the pro, the comp m4 even feels more robust to me personally. not to mention battery life, having a really common battery and more illumination settings. either optic would serve you well, I think it boils down to customers price point, if you can afford the comp m4 series, get it, if your budget is lower, get the pro.