I've been using Slingbox for almost 6 years now, and while it is very useful, it isn't a replacement for cable. All it does is take the output from your set-top box and stream it via the internet so you can watch on your phone/tablet/PC. It works well so long as you have a fast, stable internet connection (particularly upload speeds), but it's kind of useless if you ditch cable lol.
I've never used Sling TV, so I can't speak to that, sorry.