According to this article posted today, Fire TV has the edge over ROKU
Amazon Fire TV - 34 million active users
Roku TV - 29 million active users
Sounds like Roku is embedded in more Smart TVs than Amazon but Amazon has aggressively moved into Smart Tvs as they partnered with Best Buy to sell Insignia and Toshiba tvs with Fire TV built-in.
https://www.engadget.com/2019/05/15/amazon-fire-tv-34-millio…
-AJ