24 hour system makes a lot more sense, but I have been using the 12 hour system since I was a small little caterpillar, and it is hard for me to think in the 24 hour system. I even "convert" time from 24 to 12 to better interpret it.
For some reason, "pm" flips the "Panic switch" in my mind, it indicates that half the day is already over and if I need to get stuff done I better hurry up.
I guess that same effect would be induced for something like "sixteen hundred hours" in the 24 hour system, but I'm not used to it, so I cannot say.
I vote for 12 hour system simply because I have been using it for too long. If you ask me which system makes more sense, then it's the 24 hour system.