Studying the MCP 6500 and 5500 I have in front of me there are 4 differences (well 5 if you include the 6 v 5 number on the front)
Black v Grey-green background
Shiny v Matt aluminimum finish
Stronger and longer hinges v short ones
Shiny magnet v black magnet
There does not appear to be any difference in the screen covers - but I am not taking them apart to verify this.
Sadly I no longer have a MCP 5000 to compare them to, but my recollection is that they are similar mouldings and cases - the button size and wheel being the obvious differnce.
Finally my MCP 7000 (aka Beolink 7000!)
has so many differences that I can't begin to list them!