I think it really depends on what the situation is. Obviously if the characters are seperated longs posts are better because what they're doing doesn't immediately affect the other characters. Also if there's not that much going on, putting up a long post to get the story closer to action of some kind I find to be very useful. When it's in the middle of action or conversation short ones make more sense as, like Angel said, you can reply in time like a real conversation, and during action every small movement could have drastic effects on what the other people would do. If you write down a whole fight scene yourself, it's like saying that the other person wasn't involved at all. Ok now I'm ranting a bit but that's just my 2 cents worth.