I understand that you are interested in the differences between the Slingbox PRO HD and the 500. Well, the 500 has WiFi and HDMI capability. It also has all of the cool new My Media features, and it's own physical remote which you can use to run the OnScreen Setup (HDTV required) right on your TV! I noticed that you mentioned that you would like to change channels through the Slingbox without interfering with viewing at home. The PRO HD does have a built in tuner which would allow you to connect a coaxial cable and scan for channels independently of your cable box. However, you can also use a dual tuning cable box to view independently at home and remotely with the Slingbox.
Here are a couple of links with some additional information regarding your inquiry:
Hope this helps!
The Sling Moderation Team