Zef - they're all good. (500s are better of course...)
Buy the '77. After a while you will feel the urge for a second, then a few years down the line you'll find yourself with one of each!
Seriously, the '77s were split into two:
The F had 4-1, sharper styling and disks all round
The K had 4-4, classic styling and a good old drum on the rear.
Go here for more info:
http://sohc4.us/?q=node/5