I have the M4 and M6, but no M3 yet. Between the M4 and M6, I like them both. The M6 is shorter, but the M4 is skinnier. Some people don't like the "head-heavy" feel of the M4, but I don't mind. Of course, I find the shorter size of the M6 easier to fit in a bag, or to carry on a belt with my BlackHawk pouch (only tried that once!).
Paulr brought up a good point a few weeks ago when I asked a similar question (M4 vs M6)--hopefully I'm not quoting him out of context. Basically, for my purchase, I was planning to buy one or the other as a neat toy (that is, not out of any real need), so I, too, brought up the cost of batteries between the M4 and M6:
Paulr: "I'm sorry, but those questions just seem surreal. You're going to buy a $300 flashlight as a neat toy, and you care what the batteries cost?"
Paulr then provided some other info on throw that I had asked that isn't relevant here. /ubbthreads/images/graemlins/smile.gif Of course, for people who need to use the brightest, toughest light for their situation, then the battery costs become incidental(!).
But I agree with the posts here--if you want the brightest, you might as well skip everything and go straight to the M6. /ubbthreads/images/graemlins/smile.gif
Me, I'm planning to get an L6 eventually, and then use the KL6 on my M4 and use the Turbohead on my L6 body. That way, I can also run Pila 168s batteries in my M4 with the KL6. Lots of versatility (and interchangeable parts) between the M3, M4, and M6.