Owen owned a dirty, off-grey colored protocol droid. He purchased a gold protocol droid. Considering how common they are in the films, it's hardly a leap of faith to understand why he wouldn't consider them being the same.
Hmm... I'd have to go with Empire as my favorite of the trilogy, though A New Hope is fantastic in its own right. The binary sunset gets me everytime. ROTJ, while a good film in its own right, is the weakest of the five films thus far, in my mind.