Grab a copy of the DVD Advanced Kalashnikov Rifle Gunfighting by Infidel Media Group from One Source Tactical; there's a simple first-hand test of the various calibers' penetration vs. concrete block at the end. Plenty of other good information too, even if you shoot something other than an AK. (If you can review a copy of Sonny Puzikas's out-of-print Beyond the Firearm DVD on Kalashnikov rifles, he goes into the specific virtues of the 5.45x39mm round; it has a void in the center of the slug that maximizes its terminal effect.)Between the 5.56 and the 5.45 which one is better? I know the 5.56 is what the M4 carbines shoot and the 5.45 is what the AK-74 shoots.
Those two...I'd vote 5.56, simply because its way more available.
I want to know where people are seeing these .308 uppers for AR-15s...5.56 simple. Here's my reasoning behind it. You can get the m193 which is better at soft targets then the m855 round is, you can also get other way more affective rounds. Also if you get a AR 15 chambered in 5.56 you could get another upper utilizing a .308, 6.8, 6.5 or other rounds. Plus if shtf does happen, you have military also using this round and two of the rounds I stated above. But in the end, Do your research, see what rounds are prevalent in your neck of the woods. And make your choice from there.