6.8 SPC for me, hand built (by me) upper using Daniels Defense 16" 1:11 SPC II barrel, 110gr Black (VMax bullet). Performs similar to 110gr OTM, but shorter neck length and frags a little faster than OTM which is more barrier blind focused. It has demonstrated to not exit deer typically (usually penetrates both lungs and heart, gets caught in opposite side rib cage), but is quite devastating on internal organs. Shoots sub-moa with NO flyers.
Over on the 68forums, there's is a really interesting view one can get from hunting uses for deer in the 100-200 lbs range and hogs from 100lbs up to 300 lbs with that bullet. Although they tend to prefer 120gr SST more for deer because it typically exits (blood trail) and hogs because it uses a denser led alloy that does better on thick shoulder bones, it would seem that load is well suited to bad guys based on it's performance in mid-sized deer (that and it's the same bullet Hornady once sold in their 6.8 SPC TAP line).
I've found that training between my 5.56 and 6.8 uppers that practical speed is nearly the same, unless it's a gaming gun (like 3 gun, shooting mouse fart .223 loads), there's little difference in recoil / system weight, but notable advantage with full caliber projectiles shot to shot. It's about how fast it incapacitates for me rather than a question of lethality.
Cost of the "good stuff" is literally about the same between the two cartridges, but 5.56 is much cheaper for training due to mass cheap FMJ. 25rs for 6.8 spc mags vs. 28 rds for 5.56 mags in a standard sized mag (use Larry Vickers thumb nail rule for both calibers to facilitate admin reloads / reliability of feed).
Maybe that is overkill, but so is security film on my patio sliders, windows, reinforced door frames, alarm system etc. What is most valuable to me is my wife and children, so the more effective I am, the more likely I can keep them from harm. Wife runs a 8" barreled CZ Scorpion Evo 3 in 9mm, why? Because she can't effectively use an AR, so that's about as much gun as she can blend speed / accuracy at her skill level. 35rds of 90gr Leigh XD +P+.
My rifle caliber / load and handgun caliber / load choices are based off of the attached documents, that's just what I was able to find with the most substantiated testing / evidence beyond anecdotal end user accounts (which shouldn't be dismissed outright, but can't be the basis alone).