A Roku TV has a wider array of available "channels" than ordinary smart TV's.
For instance, if you're interested in AcornTV (BBC videos), you're pretty much out of luck with most other smart TV's.
And when they do offer AcornTV, the interface is garbage. For instance, Amazon FireTV's implementation of AcornTV is actually worse than useless.
That said, if you're looking for something to surf the web on your television, Smart TV's have Roku beat. There is no web browser on Roku, but this is by design. A web browser makes it too easy for hackers to get into a smart TV. The lesson is, don't surf the web on your smart TV. Norton doesn't make a product to protect them.