I haven't seen a smart tv where the smart features are any good.
I have a Roku 3 and it works great. Technology moves quickly though and when the Roku 4 comes out, I'd rather upgrade my $100 Roku box than my $1000 television.
I will likely buy a Fire TV. We have a two television sets and because of the way one is mounted on the wall, it's a pain to connect and disconnect the Roku. So for me, it makes sense to have a second streaming device and for $100, this one seems like a pretty good deal.
That said, I wish they would incorporate some DVR capabilities. On the Roku, I can get streaming versions of all my local TV stations. It would be nice if it could tell it to record shows from the streams.