Quoted:
If i have to "put someone down" It will be with something heavier than a .223.
Leathality will be very close between the 2 out to 300 yards or so. I've chrono'd both styles. Very litle difference inside 100yrds.
View Quote
Actually, there is a fairly significant velocity different between the two barrels (about 200 fps). The key is when they fall below 2500 to 2700 fps, which is the FMJ round quits fragmenting. I have also read that the effectiveness of SP bullets is reduced at that velocity level as well.
While it is true both 20" and 14.5" will both fragment out to 100 yards, the M4 will not go much farther, while the 20" will get almost to 200 yards before it drops below 2500 fps.
As to the lethality of the .223 I have no personal experience, but I can tell you that the .308 is overkill. Not that overkill is a bad thing, but I am sure something with less recoil and which weighs less would do the job. The trajectory of the 7.62x39 is going to limit your range, so it is not a real great alternative.
The .223 is certainly lethal, and it is going to be easier to shoot accurately than heavier calibers. It is not going to make the horrendous holes that .308 soft points will, especially past 140 (M4) to 200 (20") yards. But you can still put a hole through your target, and I imagine even a little hold would suck.
Justice_Keeper: I have both, and I like a lot about the M4. It would be somewhat easier to use at close range and it is a little handier. But I always find myself taking the 20" to the range. That could just be a function of how much I like the scope I have on it though (3xACOG). I also like the feel and look of it better, but that is entirely subjective. My 20" seems to be more accurate, but that should have little to do with barrel length. You can not go wrong with either the rifle or carbine.
Make sure you handle an M4 before you decide to get rid of your rifle.