What are they?
Orb is software that lets you stream your PC over the Internet. If your PC is connected to a TV input, like a Media Center PC, you can stream your TV live.
Slingbox is hardware and software that can stream your TV over the Internet.
What do they do?
Orb will stream any media content from your PC, so if you've got DVD's or music on your hard drive, Orb can make it available anywhere you have an Internet connection. Orb is free software, but you have to have a PC capable of connecting to your TV if you want to watch live TV via Orb. Orb uses Windows Media to accomplish the streaming and will stream to just about any device that will run Windows Media, including Smartphones and PocketPCs.
Slingbox will stream your TV and related inputs (like your DVD player, cable box, pvr, etc.) over the Internet as well. Slingbox units run $200 to $250 depending on where you find them. Slingbox still requires a PC, but the PC doesn't do much work. The Slingbox hardware does most of the work. Sling Media uses it's own player and has a version for Windows Mobile devices. The Windows Mobile software runs $30 but is currently free with a new Slingbox purchase.
How do they work?
Since Orb runs on your computer, it can take a lot of resources. I have a Gateway AMD-64 2.2 gig machine with a gig of RAM. If I'm streaming TV, the computer isn't terribly useful for much else. To use Orb, you login to an Orb website. The website connects to your machine and presents a an interface with TV, Videos, Music and other media items on your machine. To watch live TV, you select a show from a TV Guide like interface and click play. Windows Media launches and starts streaming the show. To change channels, you go back to the web page and pick a different channel.
The Slingbox uses a local app to connect to your Slingbox through the net. Once connected TV starts streaming right away. Selecting the remote pops up a remote control to manage your TV with. My remote looks just like my Brighthouse digital cable remote, so it's very familiar.
Which one is better?
Frankly, neither. Either. Both. Both products work just fine. I haven't test the Slingbox mobile client but my experience is that Orb works better on slower connections. I can use Orb on my Windows Smartphone with GPRS and still get a TV experience. Is it great? No, but it works. I can see who's winning the football game.
I have digital cable at home so I have a program guide built in as part of the cable box. When I hook the Slingbox up to normal cable, it can be a little tough to figure out what's on. I tend to use Yahoo!'s program guide at that point.
Slingbox is VERY easy for non-computer people to figure out how it works. My kids can run the Slingbox without any help. It's also easier to channel surf with Sling.
Orb has a built in program guide and inbuilt recording options like a DVR. It's easy to figure out what's on but a little tougher to channel surf. Changing channels takes longer because the new channel has to be streamed into Windows Media.
Both products support favorite channels and a host of other features. I've really just focused on core features here.
Bottom line, you won't go wrong with either product. If you already own a high end PC, a tuner card and Orb may be a bit cheaper. If you just want to push out your movies and music, Orb will do that for free. If you're not real computer savvy or you don't want to tie up your PC with Orb, get a Slingbox. You won't be disappointed with either one.