whiteswan,
I've played around with this a bit, and read as much as I can. I don't have all the answers, and I have an older Sling box myself, but maybe this will help...
All new products, features, and updates are via the web-based slingbox viewer. The old software you installed on your PC still works for old slingboxes, but they're not updating it any more, so it doesn't support new sling boxes. There's no need to. The web plug-in works without having to install a new application on your PC - so it'll work on your PC, friends PCs, work PCs etc. There's a web pop-out player that looks a bit like the older desktop application, and you can set this as the default if you want. There's also full screen, which is what I usually use.
The slingcatcher hardware hasn't been supported or updated for a while (you can't even find it now in the UK). I get the impression that it can't be updated - which is probably why they dropped it even though so many people seemed to love it. (Have you seen their forums?) That's the thing I've learned about Sling: with older devices, you must check compatibility, because the codecs get improved over time (see Wikipedia) meaning the newer hardware has wider compatibility, and there are things that the older hardware just can't connect to. That said, the Sling Catcher can connect to some devices that it's "not compatible" with if you configure it manually over a home network. It's worth a try.
I'd love to hear how Sling works for you over a 10MB upstream connection - I only have 350kbps upstream!
Cheers,
David.