If you can reliably stream at 3.5 Mbps or better, or could obtain such speeds by upgrading the Internet connection at the source and/or viewing location, then the 350 or 500 will give you a better picture.
For 720p content, PRO-HD is limited to 30 fps; the newer boxes can do 60. On 1080i or 1080p24 content, PRO-HD dumbs it down to 1920x540; 350 and 500 give a full 1920x1080. In addition, during sufficiently fast motion, PRO-HD drops some frames, regardless of streaming speed.
I happen to have a PRO-HD and a 350 connected to the same (Verizon FiOS DVR) source and could post comparison clips of the same footage. Let me know what kind of content, resolution and data rate you would like.
However, be aware that the newer boxes lack some features present on PRO-HD. Before upgrading, confirm that these are not required in your setup.
Thanks for posting in the forums! In addition to the above post. The Slingbox 350 and 500 also have internal IR emitters, which eliminate any physical connection of IR blasters to your set-top box.
Below is a link to a page from Slingbox.com that compares the Slingbox 350 and 500:
Have a great day!
The Sling Moderation Team