Storing 500GB on S3 would cost me over $60/month, which is pretty hard to swallow.
ATM 500GB is a pretty large in terms of options for consumer storage. Good thing is that the prices should continually go down. Bad thing is you'll continually be accruing more and more photos :).
It's a great question though. That 500GB of photos has some real value to you. I wonder what that is. And what the risk of losing it by backing up at home vs S3 vs home + S3.