I have a shield TV 2017, and I would rate my FireTV as performing 25%+ better. I prefer the interface, the eco-system. I bought the Shield TV to try it and for google app store compatibility only.
I consider the FireTV to be the best piece of AV gear I own. That is saying something, considering the other gear in my stack. It's just seamless. The only thing stopping it from being perfect, is Google Play Store.