Well, after owning my M6 for 2 days now I have to say I really like it a lot. The output is indeed superior to that of the M4's lamps, in regards to brightness and whiteness over the lifespan of the batteries, and it is the most solid feeling light I have ever held, however, I have to say that as much as I'm liking the M6, I still prefer the M4 or my M4 Frankenstein if you will. I feel I have at the very least slightly larger than average hand size and despite this, the M6 still feels quite large and bulky in the hands, and attempting to twist the tailcap for constant on with one hand is much more difficult to do on the M6 as it is on the Z41 on my M4 Frankenstein.
I think the longer, thinner body of the M4 feels much better and more comfortable than that of the M6. Also, as I pointed out in my OP, although the situation may never arise, I like the fact than if I ever require a battery change in field, I will have no issues doing so with the M4, whereas with the M6 your SOL. This in my mind is a huge advantage, especially depending on the line of work you may be in.
All in all, the M4 (or 9P Frankenstein if you will) and M6 are great lights, but even though I feel in the minority here, I still prefer the M4.
A real M4 Devestator is going to be my next purchase, I don't care how long a have to save and eat beans