They just announced today that Sling now works on Chromecast. Not sure that will matter for me.
My story is very similar to yours, I went with the 3-month sub to get the deal on a Roku 3 because we had a few Roku 2s that I wanted to upgrade. Got out of a TWC subscription when they ended my promotional rate after a year and would not offer another. When I got to the store to drop off my equipment, then they were ready to deal and I said too late.