Actually... Nvidia's GeForce Now figured this out first in November... seems like Google just copied their workaround.
GeForce NOW Streaming Comes to iOS Safari
I much prefer their model because it doesn't force you to buy games again, that will only be tied to the Stadia store. Nvidia let you link up to existing stores (like steam, epic) and play your existing purchases, platform agnostic.
and they have a free tier. That's very consumer friendly.
I like that as well. Though this is why NVidia lost so many developers.
I also found that with GeForce now, when I was testing, I couldn't tell it was streaming. It played as if it were local. I tested on my Nvidia shield and on my mac mini. Though my primary method of playing is locally on my PC.
I can't stand controllers and so I prefer a keyboard/mouse.
But comparing the two services, Nvidia feels like a PC copy of the game and Stadia feels like a console copy in terms of settings and options. It feels like on stadia a keyboard and mouse are secondary to the controller. Not every game supports them. But in those that do people say they mostly work well.
Keyboard/mouse seems primary on GeForce now though in my testing I used the shield controller but the game seems to favor kb/mouse.
Nvidia has a free tier that can have a wait. It streams at 1080 and I was able to max all settings and it played smoothly. Their $5 paid tier eliminates the wait and adds ray tracing. If you have the 2019 shield with it's AI upscaling this will come pretty close to looking like 4k with no performance hit.
Stadia streams 1080 for free, as long as you buy the game from them, and their $10 paid tier ups that to 4k. Though no ray tracing. I've read a review that said it wasn't true 4k but I don't know if that's correct.
Neither service supports non standard resolutions which is a deal breaker for me. As all my monitors are non standard resulting in a smaller image.
I'd love to test on xbox series X but then I'd have to buy another copy of the game.
Another downside is stadia has now gone down 2 days in a row. If that happens when you want to play you're SOL.