No difference because the 6442 and 6445 are identical controllers. The only difference is the display. The 6445 package has firmware available which allows for the rotary to be independently dedicated to the U axis. The 6445 keypad is worth every bit of the small difference in price. I do not like the 6442 keypad at all. Moving the z axis, for example, is just annoying. The menus and resolution on the 6442 are also not as good.
There are additional options now in the 644X series like a full touch screen as well.