I am in the process of dumping cable and going to Roku + Sling/DirecTV now/YouTube TV (not sure which yet). I'll still have most cable channels and reduce my monthly expenses by around $50.
I did a lot of research and reading about the various options. Happy to share what I found. Did you have a specific question about anything?
Roku is a stand-alone content source that comes with a remote and dedicated apps for most things you'd want (Netflix, Hulu, streaming TV) and its own free "Roku Channel" with many free movies. It's user friendly and intelligent - you can search for a show/movie and it'll search all your connected apps for the content.
Chromecast is a "hub" for your TV that you control with your phone. You cannot use a Chromecast without a smartphone. You find what you want in your phone and, if the feature is available, "cast" it to your TV from your phone. Once the content is playing you can do whatever you want from your phone again - your phone is the remote here, and not everything is cast-able.
I went with Roku myself - I liked having a dedicated remote and a ton of support for so many various media sources. Plenty of other neat features depending on which model you get and what you need.