This has been a question of much discussion in the various forums. Do a search and you will find numerous posts and opinions on the merits and demerits of the various digital scanners.
One of the biggest things to consider in buying a new scanner is your scanning style. Like Aaron, I have both the 500 and 396. My scanning style leads me to prefer the 396.
The 500 has some nice bells and whistles, but they don't offset the fact I cannot listen the way I want to with the 500.
Just do some research in the various threads, determine your scanning style based on your area, rate the ratings and make your decision.
While I'm at it, welcome to radio reference. This is the best place for your scanner info on the net.