They're the same electronically. I think I would order a blue one over a silver one, since as you point out there is more contrast between the case and the screen, to help keep the eyes focused on the screen better. But when I held one of the silver ones at my local Borders store a week ago it was fine.
I know I like the dark case of the 500.
So I'd suggest not worrying about buyer's remorse on the case color -- if you hate the blue one, you can send it to me!
