Hi mate
I guess it all comes down to spec while the 500hd has a better CPU and can play divx etc unlike the 800. The 800 has a small screen telling you whats going on and you can use a internal hdd.
Early 500hd clones had tuner issues and needed hardware modifications. So ask your seller if these have been carried out or if the 500 is a later revision with the modifications been done at the factory.
Ill be honest when a new clone comes out it always best to wait a bit as there is always a few gremlins. I don't own a 500hd but the above info is fact.
Ferret