The new ones (Dish 500) have a new encryption technique. Dish is mailing them out to subscribers now, and one day they will probably turn off the old encryption stream, locking out the hacked recievers.
The prices of the used 721's are going to drop once the new promotion for the 721 @ $249 rolls out soon. One could very well find used ones for that much if not below $200 in the future once they have been out for a while unless it would be a short lived promotion. This would drive the price down on the used 501/508/510 receivers even more.
Those receivers are the same exact receiver if it indicates the same model number.