I am lucky enough to be in both the (soft-)launch of Google Stadia and the beta of project xCloud from Microsoft. Some thoughts on the differences between the two:

• Stadia works a bit like Xbox Live. You pay a monthly fee, and get a couple games added to your library each month. You lose access if you lapse the subscription, but regain them if you renew again. Xbox costs half as much if you pay annually – not sure Stadia offers that yet.
• xCloud has 2 options at the moment – stream games from their servers (similar to Stadia), or stream your own games from your own console (this requires opt-in for beta testing the OS on your console).
• While both work best on 5 GHz band wifi, Stadia gives a more general “you’re good!” connection status, while xCloud will specifically tell you to get on 5 GHz for the best performance. The difference here is you can keep going on Stadia, and xCloud will generally lag and shutdown the stream to change the connection.
• Playing a console game on a phone isn’t terrible, but is not yet ideal. As it becomes more widespread, I would guess games would adapt their display settings for this, but right now, it’s a lot of tiny text menus.

Of course the biggest difference is the platform buy-in (and I suspect this will be the case with Nvidia GeForce Now as well, if they can stop hemorrhaging games). I have nearly 15 years of game saves and purchases and achievements on one of these platforms. One of the best reasons to set that aside would be ubiquity – if I can pick up a game and play on any screen¹, it would be hard to beat the convenience.²

Google has that opportunity, if it wants. The next play is for your triple-A game library to follow you wherever you go. They’ve just got to move quickly, before someone else beats them to it.