Quick disclaimer, I am a Google shill who has tried both services. Overall I prefer GeForce Now slightly, I think Stadia does have some advantages. Whether or not they're worth it depends on your use case. GFN works great to supplement a gaming PC when you're traveling or laying in bed, but has some deal breakers that keep it from being the main way I play games.

If you use the free version of Stadia, it's the same resolution as GFN. If you're a monthly subscriber, the picture quality is much better than GFN because GFN doesn't support resolutions higher than 1920x1080. GFN is REALLY frustrating if you have a 1920x1200 or 1440p monitor. Because of this, GFN can complement my gaming PC (like when traveling with a crappy laptop) but never replace it. Stadia probably could replace my gaming PC if I was willing to buy the subscription.

No waiting period (its rare, but there's occasionally a wait for GeForce Now even with the founders edition)

Games don't disappear from the service like they sometimes do with GeForce Now (granted, if a game you bought disappears in GFN you can still install locally so this may be a win for GFN as long as Steam's cloud saves don't glitch on you)

Stadia doesn't require you to install a 200 megabyte app (assuming you already have Chrome installed)

GFNs video clip feature is a bit buggy for me compared to Stadia although I haven't used those features enough to tell if GFN is consistently glitchy or if I'm just unlucky with the NVidia overlay

The UI for GFN feels clunkier to me but I suppose that's personal opinion. It feels more like VPNing into a really locked down PC, where you'll hear Windows sound effects when there's errors and if a game crashes you're stuck with a Steam error and you have to restart the whole app and wait in a queue to play again. Stadia feels a bit more well-integrated to me.