I don't own a SlingBox, so I can't comment on those.
I DO own a Vulcano (same concept, different company) that I really like. I opted for the Vulcano specifically for the fact that the PC software is free and the Android app is only like $16. Plus, I didn't have to buy it more than once, since the app purchase was saved to my Gmail account, and all three of my Android devices were linked to that account
The only complaint I have is that the Vulcano steams the signal out to their media server, and then back to your device via the internet, only PC's can connect directly over the local network. That means you need an internet connection with a beefy (read 3+ MBps) upload speed, or you're going to have really choppy low quality. That didn't bother me as bad considering I have a 10/10 fiber connection, but trying to use it from a Cox connection with a 2.5 MBps upload rate was terrible.