As I recall, my gripe with Stadia was the limited graphical capabilities. I don't recall if the graphics settings were locked but I remember only being able to play games on medium settings and not being able to achieve any better performance than a GTX 1080.

Meanwhile, your XCloud stream is hosted by an equivalent of an Xbox Series X, which while a console-level experience, the games were shinier and prettier than Stadia.

And otherwise I've found their streaming quality, performance, and latency, to be about equal (not the best) although XCloud continues to improve to this day.