I have both.
The 500 has hdmi aswell as the ability of apps such as youtube in built. For example you may be watching a video on your phone on youtube and you can send it to the TV via your slingbox.
If you are in the USA there is also sports scores ( sorry Iam in the UK so a little vague on that feature).
Ultimately Iam sure the 500 was developed with thinking of adding many other apps but it did not materialise.
The quality of stream is the same.
Hope this helps