M16 has less recoil due to smaller cartridges which is more comfortable for operator. Also, 5.56mm bullets are more likely to yaw in the body thus causing more damage than 7.62mm which might pierce through it.

On a side note: it's said that 6.5mm bullets are more effective than these two. Those were being developed in various European countries but under US influence they had to adopt 5.56mm as a standard.

However, I wish that she had her hands on 5.42mm AK-74M which is a standard issue rifle in Russian army. It would be the best of two worlds.