On TV Stadia definately blows Geforce away for me...of course I am playing Geforce on a FireTV and MiBox and not a ShieldTV so I am not the best person to compare.Both for me. On my 4k TV via a Chromecast Stadia looks essentially native. On my PC monitor (1080p) it looks great too. GeForce Now looks like a Steam Link stream. Like it's washed out.
The both look great on my monitor, I just find that Destiny 2 and Division 2 that I am playing on Geforce really look great in the color contrast department, just wish Geforce was more stable.