The biggest difference (and problem for Stadia) was that you have to buy Stadia games as “Stadia games”. You cannot transfer them out, they are locked into a platform run by a company which is famous for shutting services down rather often. Why would anyone choose Stadia then?
The nice thing about Geforce Now is that you can just run your already bought games on it. Just connect your account to Steam/Epic and you are good to go. You are just renting their hardware essentially.