Ahhh, namedy you have got my on my nerd subject of the week! 
I have done some RESEARCH into this.
Well, I Googled it. That counts, right? 
So, from what I can work out, paint has stuff in it to make it gloopy, and stuff that gives it colour. When they make DIY paints (non-trade ones), they basically think everyone is incompetent, so they make it with something called "false body", which is basically a gloopifier that makes the paint easy to pick up and work with, so inexperienced people don't drip it all over the carpet. When they make a trade paint, they assume that people know what they are doing, so they use "true body", which basically means loads of pigment, so you get more opacity.
The diamond finish is supposed to be reaaaaaallly tough, very wipeable etc.
So I'm going for trade with diamond finish. 