Here's how it looks to me. The two units are remarkably similar, though if the published specs are to be believed (I doubt it), the 575 weighs about a 1/4 lb more and differs slightly in dimensions from the 501. Because the 501 isn't sold in Sony's own store, though it's available at Cosco (if not in other similar venues), I'm led to believe that it's a limited edition intended only for the buying-club market or similar outlets. I've seen other companies distribute their products like that, changing a few unimportant particulars to make sure that the units are not identical with their main lines.

Ed