I don't want to stir the pot any more than it seems to be being stirred now, but if I may:
M4 Carbine.net may be able to give you much better answers to this question as the entire site seems to be dedicated to M4's and AR's.
There is a comparison chart there that will lay out all of the answers for you. What it will come down to is,
What do you intend to do with the rifle, how much money are you willing to spend and how much research can you handle?
In the end of a camparison like this, it would seem for a couple of hundred dollars more you will be far better served with a Colt 6920.
I do not want to fan the fires any hotter than they already seem to be, but there you have it.