Oh, boy. Here we go with the .556 vs x39. Let's face it: both great at going boom. There are enough examples of these two calibers in active military use all around the world that it's safe to say they are both effective at ventilating your target.
"But the x39 can blow through a cinder block!"
"But the .556 is good out to 600 yards!"
"But the x39 has more stopping power"
"But I can carry more .556"
"But .556 can deflect off windshields"
"But the x39's trajectory looks like someone lobbed a football at 100yards"
Come one, guys, can't we all just get along? Why not just have both? If I had a Tavor, I'd love it. If I had a "super-ex-pee-alidoshous" tactical CZ, I'd love it too. In a SHTF scenario, use what you have and best of luck. I wouldn't want to be on the business end of either rifle.