They are imo very different services and both have trial time.
Test them both and decide for yourself.
I personally prefer nvidias service because I actually play games in my steam library. Stadia has few games and you're always forced to use their services. No way to play locally even if you have decent hardware
Ah, I got it through Steam as I already had a Steam account. Been playing around with HL2 and the input lag is there but not horrific. I probably wouldn't use this for multiplayer. Just need to source a gamepad of some description.
Test them both and decide for yourself.
I personally prefer nvidias service because I actually play games in my steam library. Stadia has few games and you're always forced to use their services. No way to play locally even if you have decent hardware