1: Exact Penetration: A bullet that will expand its energy within the target and not over penetrate
2: Recoil Factor: The larger the round the more recoil. In a serious of larger burst accuracy will suffer due to larger recoil cycles. Larger rounds such as the 7.62 have their advantages as well as their disadvantage this being one of them.
3:Round Availability: A common round that one will come in contact with frequently.
My round of choice is the 5.56 Rem