As far as pay goes, where I work, in the upper midwest, everyone, including unloaders, on the early morning - preload shift got the extra dollar. (At least that's how it was when I worked that shift. I have been a PM air driver since 2004.)
Though you did not ask this, for what ever its worth, I was an actual preloader, and never unloaded. Anyway, to make a long story short, I found the worst thing about being a preloader was dealing with the drivers who were whiny. That's basically what got me into air driving. As an unloader, you won't have that issue.
Unloading on twilight and sunrise is the same in that you are unloading the semitrailers, though I can think of a couple of differences: On twilight, the trucks often have a more stable load because they have just come from the originating business. With preload, the packages have been transfered at least once, often multiple times, and the load quality is not nearly as good. So, on the unload during preload, you need to watch out more for packages falling on top of you as you are working. A 50lb box of screws falling on you can hurt you...so you do need to be careful and aware of things. The other difference I can think of, and one that makes the preload better, is that you don't have to go inside a trailer that's just been opened up after its been traveling sealed in the hot sun during the warm weather months. Though do be aware that unloading, it still is somewhat hotter over the summer and a probalbly a lot colder now in the winter, if you building has any heat compared to other work areas.
All the best to you and keep us posted.