I have a C3. Bought it from Midway for $355. At the time, that was one of the best prices. Being a 3rd generation, I get the 50,000 hr battery life.
The differences are small. The C3 is water proof to 17ft, the M3 to 75ft. The M3 has lense covers, and has that triple lanyard for the windage, elevation and battery covers. The C3 has the 2nd generation style body.
Some people claim there is a difference in brightness. I call
on that.
One of my biggest concerns was dot brightness. It gets very bright and very sunny here in Florida, espcially around noon in the summer. Much more so than northern states. Virtually every red dot I looked at would wash out except for the Aimpoint, and the EOtech. I chose the Aimpoint because I like a single dot better, had better battery life, 10 year warranty, and I like the on switch on it better.
I compared the C3 to other Aimpoints, M2's M3's...and I see no difference in brightness whatsoever. And like I said, this was an important consideration and I am picky.
I believe the difference in price is for more waterproofing, perhaps some more shock resistance. This isn't to say that the C3 is a POS. It will take severe abuse. You just can't go scuba diving with it like an M3 on a SEAL's rifle. You could snorkel though! lol...
Edited to add: A few folks say the M series have different coatings. I'm not entirely sure of that. They look the same to me, but it might not be a visable difference. As for your concern. Get the C3. It will do 99.9% of what YOU need it to do, and will be more robust and superior to almost every other red dot out there. In my opinion (I'm not a cop or in the military) this optic is probably good enough for "duty" work.