Personally, I would rather be shot by neither, but a Special Operations doctor spent some time going into detail about why he would rather be shot by the 7.62x39mm round from the ubiquitous AK-47 over the smaller 5.56mm round fielded by many NATO forces.
“Admittedly, I’d rather not be shot with either, but if I had to choose, I’d take a round from the AK-47 over the M4 any day of the week. To add a caveat to that statement, I’m talking from relatively close range here — say up to 150 to 200 meters.”
Bold statement, but he backs it in an article on Business Insider with physics and his personal experience. Simply put, while the 7.62×39 is bigger, heavier, and slower, the zippy 5.56mm has a tendency to yaw significantly when impeded or given sufficient depth of soft tissue creating some nasty wounds (which the XSTAT would help with significantly).
The doctor also speaks to how the 5.56mm requires velocity to use its primary wounding mechanism. If the round is too slow (like at distances past 150 to 200 meters) or does not have sufficient tissue, the zippy rounds “zip” through a target, leaving only a small permanent cavity and bruising (a common complaint prior to the new M855A1 round, which has significantly different wounding patterns).