I have heard anecdotally of multiple instances of 10.3" Mk18's shooting Mk262 ammo being lethal out to 600-700 meters. There's also many videos on Youtube, etc. of people slapping steel out to 500 or 600 yards pretty easily.
My general rule of thumb, whether right or wrong, is that the 10.3's 10.5's, 11.5's etc are great for 300m or less, thick east coast woods, urban areas, in and out of vehicles etc.
For a "do-all" rifle I think 14.5 is hard to beat, or the newer trend of 13.7's with pinned Surefire flash hiders. I use a 14.5 for general purpose use and can consistently hit steel out to 600 yards, and have gotten regular hits out to 800 yards using 77gr match grade ammo. It's still short enough that it's not a hassle to carry it around, and would be fine for CQB work in a pinch. Out here in the wide open west having a little extra 'oomph' for the longer range shots is nice to have.
I think the only advantage a 16" has is that it makes it easy for the average joe to swap out muzzle brakes, flash hiders, etc because they're not pinned. Muzzle velocity difference isn't significant enough to justify the extra length.
Obviously the shorter barrels have less muzzle velocity, but I honestly don't think the feral hog, deer, home intruder, etc. would be able to tell the difference between getting hit by a 10.3 or a 14.5.
Edited to add. I also think it makes a difference if you plan on suppressing it. 14.5" is the longest I would want if there will be a suppressor mounted. The 10.5's are also pretty blasty, and a suppressor can really tame them down.
