Thank you for posting in our online answers! That's a great question! The Slingbox 500 uses HDMI, where the Slingbox 350 uses Component (green, blue & red) cables. Both models allow you to stream in HD. Honestly, I were looking at both models in HD, I would not be able to tell you which one is using HDMI or which one is using Component cables. The only other difference is that the Slingbox 500 has built-in WiFi. The Slingbox 350 needs to be hardwired via Ethernet cable to your wireless router.
Here are the tech specs to the Slingbox 350 and the Slingbox 500:
Hope this helps!
The Sling Moderation Team
I agree. I could not tell the difference between my 500 (via HDMI) and my Pro-HD (via component). Feature wise the 350 is very close to the Pro-HD. Especially considering the strong recommendation for those with the 500 to connect component as a backup (for HDCP issues), in my opinion there really seems to be little justification to purchasing a 500 other than the need for wifi connectivity. If ethernet (wired) is a feasible option, I would go that route for the reliability.