I got my hands on one of the new Slingboxes the other day - a 350.
So I thought I'd jot down a few things I've noticed in terms of the video stream quality to maybe assist others.
I now have both a Slingbox Pro-HD and a Slingbox 350 on the same network, both connected to the same Sky+ HD box in the UK (via the component chain through on the SB350).
Since I don't use UPnP I used a DHCP reservation to fix the 350's IP address and then a manual port forward. This generally works fine, though I've yet to make it work via my 3G phone. I am able to connect to the Pro-HD (on port 5001) but not the 350 (which is on port 5201). Accessing via remote Wi-Fi works however, so it must be an issue specific to 3G or mobile networks.
Also on the same network as the boxes is a SlingCatcher, and my laptop which is a recent Macbook Pro Retina which boots into either OS 10.8 or Windows 7.
All testing was done at the Best (HD) setting on the Web player.
OS X 10.8 Web player
The difference in sharpness and fluidity between the Pro-HD and the 350 is pretty negligible to my eyes, though it may be slightly sharper on the 350.
However I noticed that the stream peaks at between 3-4mbps with the 350, but goes right up to 8mbps with the Pro-HD. This is was the case when streaming both locally and remotely.
Perhaps the algorithms have been tweaked so less bandwidth is needed? Or was the 4-8mbps bitrate on the Pro-HD simply unnecessary (I must admit could never tell much difference once you got beyond 4mbps)?
Windows 7 Web player
Here I was pleasantly surprised to see there was a marked improvement when comparing the Pro-HD and the 350. It wasn't so much the sharpness as the smoothness of the video. This really now does look like watching proper TV, whereas I've always felt that even the best streams outputted by Sling devices never quite felt as smooth as watching satellite or cable TV. The difference is now much closer.
Further, this benefit comes at a bitrate (as with OS X) of 3-4mbps with the 350 vs anything up to 8mbps with the Pro-HD.
WDTV Live (3rd gen)
WD just updated the firmware meaning this device now works with the new Slingboxes.
I was pleasantly surprised by the quality of the WDTV stream - again, it has the smoothness I refer to above with the Windows 7 Web player.
Once again, the bitrate was a modest 3-4mbps maximum for me (as reported by the network test option), though I've not tested this remotely yet, only locally.
This doesn't work with the new boxes, but for the record, the smoothness of the Windows 7 Web player and WDTV Live box was superior to the SlingCatcher for me, noticeably so.
The Slingcatcher still wins on ease-of-use vs the WDTV though - for example, in the Sky+ HD planner, the coloured buttons are used a lot. On the SlingCatcher remote you can simply press these coloured buttons and they work as expected, whereas with the WDTV you have to press the blue button to bring up the control bar and then navigate to the section showing the coloured buttons and then choose one. This is a pain! It's a shame the coloured buttons couldn't have been mapped directly, and some other button on the WDTV remote used to show/hide the control bar.
Much has been made of the removal of certain features of the new boxes - custom remote support, no SlingCatcher support, no desktop player support, no manual network configuration, HDMI HDCP limitations on the 500, lack of tuner on the 500, lack of multiple/audio inputs - but if you can look past those, from my early tests it seems like there are gains to be had with the quality of the stream depending on what device you use to view them.
I'd like to see some improvements made to the Mac Web player however - on the same hardware I was able to get a smoother stream from the 350 via Windows 7 than via OS X 10.8.