although I am not an expert and don't have first hand experience... my understanding is that getting hit by a smaller high velocity bullet is worse than getting hit by a bigger slower moving one. Sure, neither situation is great but it's hard to argue with 1/2 mv^2.
from wikipedia... for what it is worth
The M16 rifle fires the 5.56x45mm cartridge and can produce massive wounding effects when the bullet impacts at high velocity and yaws in tissue leading to fragmentation and rapid transfer of energy. This produces wounds that were so devastating that the International Committee of the Red Cross (ICRC) and many countries (Austria, Argentina, Belgium, Bolivia, Bulgaria, Burundi, Cambodia, Cyprus, Germany, Ireland, Latvia, Lithuania, Luxembourg, Mauritius, Mexico, Romania, Samoa, Slovenia, Sweden, Switzerland, etc.) considered the M16 to be an inhumane weapon.