If I am not mistaken the Military specifications for the modular pistol contract that was just awarded to Sig required an external safety. (The new pistol will be striker fired. Beretta 92 has a safety/de-cocker, and the 1911A1 had a safety.

Accidents do happen and apparently the military feels that an external safety helps reduce the risk. Also the specs required that the pistol be broken down without having to pull the trigger.

I have a Shield with a safety. I take it off when I carry and otherwise it stays on.

I say whatever you train with and are comfortable with.