I've had both, and I really can't imagine going back to the 500 over the 505. It's not just the screen, although that's a big factor -- the 505 is snappier to use, has better-positioned controls, and better battery life.
You wont even save a huge amount of money buying the 500 -- second-hand prices are strong. Mind you, the flip-side is that you won't
lose a lot of money if you buy one have have to sell it again
