The PRO is made of High strength aluminum and an M3 is made of Extruded high strength aluminum. The PRO is night vision compatible and the ML3 is not but the M3 is. The M3 has a much longer battery life.
Other than those differences, they are pretty much the same except the PRO is meant to be an affordable optic only available to LE officers and military personal who don't make much money but still deserve a high quality optic. That said, their are plenty of dealers violating their agreements with Aimpoint and selling the PROs to the public.
I own the pro and love it! It does come with the M3 manual. When it comes down to it the major difference is battery life and 30,000 hrs is more than 3 years of continuous use (longer if you shut it off when you store it) which is plenty long for me.